Earth human characters should probably give distances in light years. A light year is the distance traveled by light in a Julian calendar year 365.25 Earth days or 31,557,600 seconds long. Long distances could be given in kilo light years or mega light years, thousands or millions of light years.
Speeds can be expressed as so many light years, per year, day, hour, or other time unit. 5 light years per hour is 120 light years per day, for example. Or speed can be given in multiples of c, the speed of light, as in 1,000 c or 1,000 light speeds, for example.
Those units are for interstellar navigation, and for strategic considerations. Combat might happen at much shorter distances and at speeds much slower than light, as it does in some space opera type war stories, in which case different units would be used for tactics. But since I don't know the space war tactics in your story, or what units would be useful in those tactics, I am considering the units used to describe how far away a destination star might be and how long the voyage there might take.
If you are writing about an intragalactic war, which is a war between different governments in the same galaxy, usually called an interstellar war, you probably need to have spaceships which travel faster than light, or at least arrive at the destination in much less time than light would take to travel the distance.
Part One: Average Speeds.
In our region of the galaxy, which may be the setting of your story, especially if the story involves Earth humans who only had faster than light (FTL) interstellar travel for a few decades or centuries, the average separation between stars is about 5 light years. Distances to stars are usually given in parsecs, preferred by astronomers, and in light years, preferred by science fiction writers because they make, time, distance, and speed calculations easier.
So if your characters travel between two stars, the distance will probably be at about 5 light years if those two are the closest neighbors, and of course if those two stars are not the closest neighbors to each other the distance can be many times as great.
Suppose that the spaceship travels between two stars five light years apart. If the voyage takes one Earth year, the average speed will be a 5 times c, the speed of light. If the voyage takes one Earth day the ship will travel 5 x 365.25 light days, or 1,826.25 light days, in one Earth day, or at an average speed of 1,826.25 c.
If a spaceship travels 100 light years in 1 Earth year, it travels at an average speed of about 100 c. If a space travels 100 lightyears in 1 Earth day, it travels 100 X 365.25 light days or 36,525 light days, in one Earth day, or at an average speed of 36,525 times c.
Part two: Accelerating to a Steady Speed, and then Decelerating at the End of the Voyage.
Note in my examples I talk about average speeds. Suppose that it takes one week, or seven Earth days to accelerate to the ships travel speed, and that travel speed is 1,000 c. Halfway during acceleration, the ship will be travelling at 500 c, and 500 c will be the average speed of the ship during acceleration. So during the acceleration phase the ship will travel a distance of 7 X 500 light days, or 3,500 light days, or 9.582477755 light years.
With these assumptions the acceleration and deceleration phases of an interstellar voyage will total 14 Earth days and a distance of 19.16495551 light years will be travelled in those two phases of the journey. So if a space travels 100 light years it will travel 80.83504449 light years at 1,000 c, for a total of 0.0808 years or 29.5122 days, plus 14 more days for acceleration and deceleration, making the total voyage last for 43.5122 days.
Part Three: Accelerating for Half the Voyage and Decelerating for Half the Voyage.
What if a spaceship constantly accelerates for the first half of the voyage, and then constantly decelerates for the rest of the voyage. Suppose it travels 100 LY in one earth year. So its average speed is 100 c, reached after 0.25 Earth year of acceleration.
Supposed that a ship with that acceleration rate travels 1,000 light years. It will reach a speed of 200 C after half a year, and 400 C after 1 year. If the ship then decelerates for one year. it will travel for 2 years at an average speed of 200 C and thus travel 400 light years.
So suppose that it accelerates for two years and decelerates for too years. Thus it will travel four years at an average speed of 400 c, and a total distance of 1,600 light years.
Suppose that it accelerates for 1.5 years and decelerates for 1.5 years. It will travel for 3 years at an average rate of 300 c and travel 900 light years.
Suppose that it accelerates for 1.75 years and decelerates for 1.75. It will travel for 3.5 years at an average speed of 350 c, travelling 1,225 light years.
If it accelerates for 1.6 years and decelerates for 1.6 years it will travel for 3.2 years at an average speed of 320 c. for a total distance of 1,024 light years, which is close enough for now.
Suppose that this spaceship accelerates for 10 years and then decelerates for 10 years. It will travel for 20 years at an average speed of 2,000 c, and thus travel 40,000 light years.
Suppose that this spaceship accelerates for 20 years and then decelerates for 20 years. It will travel for 40 years at an average speed of 4,000 c, and thus travel 160,000 light years.
Part Four: Constant Speed.
Suppose that space ships don't accelerate to reach FTL speeds. Instead they instantly reach their travel speeds and instantly stop at the end of the voyage. In our region of the galaxy a star might be only 2 or 3 light years from its nearest neighbor, or as many as seven or 8 light years, but the average distance between a star and its nearest neighbor is about 5 light years.
So in your story a voyage between two stars should be at least about 5 light years long, even if one of those stars is the closest neighbor to the other. The stars in our galaxy are thinly scattered in a sphere about 100,000 light years in diameter, but the vast majority of stars are concentrated in the galactic bulge area a few thousand light years in diameter, and the galactic disc extending our from the bulge, which is approximately 1,000 light years "thick" from "top" to "bottom" and approximately 100,000 light years in diameter.
So a starship travelling through the thickness of the galactic disc in our region of the galaxy would pass close by about 200 stars or so, while a starship travelling through the diameter of the galactic disc would pass close to about 20,000 stars. That assumes that the stars are equally spaced in all regions of the galaxy, even though they are closer together at the core of the galaxy and farther apart in the outer parts of the galactic disc.
Unless your characters happen to make a voyage from one star to its closest neighbor that happens to be unusually close to it, the shortest interstellar voyage they can make in this region of the galaxy is about 5 light years. Since the galactic disc is about 100,000 light years in diameter, the longest straight line interstellar voyage your characters can make within the galactic disc is about 100,000 lightyears long, or about 20,000 times as long as the shortest interstellar voyage they can make in this region of the galaxy.
Since the Earth is about 26,000 light years the center of the galaxy, the farthest your characters can get from Earth within the galactic disc is about 76,000 light years or about 15,200 times as far as the shortest interstellar voyage.
Thus if your space ship travels at a steady speed of 10 times the speed of light, or 10 c, it will take 0.5 years to travel from one star to its nearest neighbor, and about 7,600 years to go from Earth to the far edge of the galactic disc, and about 10,000 years to travel across the diameter of the galactic disc.
If your space ship travels at a steady speed of 1,000 c, it will take 0.005 years or 1.82625 days to travel from one star to its nearest neighbor, and about 7,600 years to go from Earth to the far edge of the galactic disc, and about 10 years to travel across the diameter of the galactic disc.
If your space ship travels at a steady speed of 100,000 c, it will take 0.00005 years or 0.0182625 days, or 0.4383 hours, or 26.298 minutes to travel from one star to its nearest neighbor, and about 0.76 years to go from Earth to the far edge of the galactic disc, and about 1 year to travel across the diameter of the galactic disc.
Part Five: The Scale of Your Story.
The Milky Way Galaxy contains about one to four hundred billion stars (100,000,000,000 to 400,000,000,000 stars) so any government of even one percent of all the stars in the galaxy would be an incredibly complex government. Thus you might want to make your interstellar war fought between between governments ruling tens, hundreds, or thousands of stars, so their governments might be no more complex than that of the USA, or the Holy Roman Empire, or the United Nations, instead of incredibly complex galactic scale governments.
Thus you might want to make the governments fighting your space war rule only tiny fractions of the galaxy, and make the greatest distances traveled in your story only a few tens, or hundreds, or thousands of light years. One effect of that would be to reduce the difference between the shortest and the longest interstellar voyages in your story, and thus reduce the difference in travel times.
Part Six: Faster than Light (FTL) Space Radio?
Will your story have a FTL space radio form of communication, or will space ships be the fastest methods of communication?
If there is a space radio type of communication, you will have to decide how fast it is.
Do you want to have characters sometimes have conversations with people in other stars systems with no detectable lag in the conversation? Conversations with astronauts at the distance of the Moon had a noticeable time lag due to the three second round trip time for radio waves. Thus I guess that the round trip travel time of hypothetical interstellar FYL radio can be no more than about 2 seconds to avoid noticeable time lag in conversations.
A light year is defined as the distance light travels in a Julian calendar year of exactly 365.25 Earth days. Thus a round trip conversation between two star systems exactly 5 light years apart would have a round trip distance of 10 light years or 3,652.5 light days, or 87,660 light hours, or 5,259,600 light minutes, or 315,576,000 light seconds.
So if the round trip travel time has to be two seconds or less to avoid noticeable time lag, the speed of interstellar FTL radio has to be at least 157,788,000 c. At that speed communication over 76,000 lightyears will take no more than 0.000481658 years, or 0.175925925 days, or 4.22222 hours, and communication over 100,000 light years will take no more than 0.0063376 years, or 0.231481481 days, or 5.55555 hours.
And maybe a writer will want round trip communications between two stars to take one Earth year for story reasons. In that case the speed of FTL radio will be 152,000 c if the two stars are 76,000 lightyears apart and 200,000 c if the stars are 100,000 light years apart. At those speeds two way communication over 5 light years will take 0.000065789 years, or 0.024029605 days, or 0.576710526 hours, or 0.00005 years, or 0.0182625 days, or 0.4383 hours.
Of course if the two stars that two communication between takes one Earth are not 76,000 or 100,000 light years apart, but much closer, the speed of FTL radio will be much slower, and communications over about 5 light years, the average distance between a star and its nearest neighbor, will take longer.
If your space war happens within a relatively small section of the galaxy, the difference between the longest and shortest communication times will be much smaller, but still significant.
And of course, a writer might decide that the speed of FTL interstellar radio is not constant, but can vary according to scientific factors that the main characters might not understand, not being FTL radio operators, and thus have more leeway in how long interstellar messages might take.
Part Seven: Jumping Between Stars.
And another possibility is that a space ship might make a jump through hyperspace or subspace and disappear from one star system and and reappear in another star system without travelling the distance between the two star systems.
In such a system of interstellar travel every voyage should take exactly the same travel time, which a writer might or might not desire.
If the writer wants starships to teleport between stars they can make interstellar voyages take longer or shorter times by limiting the distances of jumps between stars. If it takes time to recharge the jump engines between jumps, or time to calculate how to jump to the desired destination, the number of jumps the starship has to make to reach the destination will determine the travel time.
Or maybe a starship can travel any distance in a single jump, but various scientific factors change how much time it takes to charge the jump engines, or how long it takes to calculate the jump factors before a jump. Depending on the roles of the main characters, they may understand those factors or be completely ignorant of them.