I will explain this as briefly as I can. I'm thinking about beginning a novel that has an MC that is born into a race of super-efficient alien invaders.
They're able to take over entire planets without the natives realizing they're there until it is much too late. The way they do this is by taking it one town/city/village at a time. The alien homeworld initially sends out a single conqueror alien to each galaxy.
The conquering alien begins by taking over a world within the galaxy that has a needed commodity to take over the other worlds of the galaxy. They take over that world by using the same selection process with the cities/towns/villages in that world. They conquer it city by city, world by world, using a combination of ingenious manipulation, their race's unique powers, and reinforcements.
Once the conquering alien has gotten the desired city to relinquish its authority to them, they call in warriors to act as law enforcement, then they establish a new set of rules for the natives to adhere to, and then finally they bring purebloods from the homeworld to be rulers. Then, the conqueror alien moves on to the next city, town, village, or world. The conqueror only has to worry about getting the natives to give up and establishing the laws, maintenance is handled by reinforcements.
I have already thought through the why, the how, and established that this method of conquest is feasible. Assuming that not every celestial body in the galaxy is inhabited and that the majority of those that are habited are homogenous societies or societies with smaller populations than that of earth, approximately how long (in years) would it take for an entire galaxy to be conquered?