I'm sorry this is such a rant in advance. It's such a broad and complicated topic that the real answer is "anyways between 1 month and 30 years." Given such an unsatisfactory answer, I've tried to pin down a few details which I think help shape that answer into something more usable for your use.
It's hard to pin down a time, because the definitions of "swordsgirl" are so tricky. If you try to train her to be a swordsgirl in general, and are using the ability to stop a charging boar as your benchmark, the answer is longer than if you are training her simply to stop charging boars with a sword.
The latter, interestingly, is more limited by how long it takes to truly understand how a boar thinks than it is limited by the ability to wield a sword.
I would fall on the classic answer of 7 years. It's not very specific, but it's probably a good foundation for your story. There is a strong popular opinion that "mastering" a skill takes 10,000 hours. If you dedicate your life to that skill (40hr. week), but allow some room for family time and vacations, that comes out to taking roughly 7 years to do. It is believed that that 10,000 hours number is related to how the brain adapts and grows, though the actual method is not known to us at this time. It just seems to be remarkably coincidental that so many guilds claim that to be how long it takes to master their skills.
For another point of view which might be helpful, consider the Japanese ranking system which grew popular in the 1600s and remains to this day. They have two types of ranks: dan (which is often translated as "step"), and kyu (which I have trouble finding a translation for, but "grade" is often used). Dan ranks are considered "master" ranks and kyu are considered "student" ranks. The flow is a bit interesting: you start with a high numbered kyu (like 30 kyu). As you get better, you move towards 1 kyu (so your first promotion is to 29kyu, then 28, and so forth). Once you are 1 kyu, you can become a 1 dan, then a 2 dan, and so forth. There is a limit to how far one can go in this way (in Kendo, nobody can proceeed past 8 dan, that's the highest they will measure).
I bring up this unusual grading scheme because their chosen scheme fits rather interesting with the topic you are looking for. 1 Dan is not thought of as a "master" in many arts. They are thought of as "minimally competent of the basics." They'll be allowed to teach and such, but it is assumed they are just starting their "steps" towards mastery.
More pointedly directed at your questions on swordsmanship, many schools of sword-arts in Asia have adopted this scale and have a rule "you never touch a real sword until you are 1 dan." Up until that point, you only use training weapons. In the most traditional Judo, the black belt was seen as the "first step" on your Judo journey, and was actually the first belt you were awarded!
From the same article, a quote which is curiously in range with the 7 year viewpoint:
On their initial interview, Matajuro asked Tsukahara Bokuden, "How
long will it take me to master the sword?" Bokuden replied, "Oh, about
five years if you train very hard."
"If I train twice as hard, how long will it take?" inquired Matajuro.
"In that case, ten years," retorted Bokuden.
However, all of that is with regard to Asian martial arts, which have a general pattern of "train until you are ready." European arts open up a different path, because they're more likely to send someone out to lean quickly in real combat. Fewer years separate starting training and entering the field.
As a result, there is more focus on tactical exchanges in European arts, little logical bits that are easier to fit into a coarse combat. It might only take a year or two to be ready to fight a beast in this way. Much shorter than the Asian approach.
However, there is a sneaky catch: the 7 year rule still holds. When facing a boar, you are facing a wise and powerful adversary. You still need 7 years of training to be able to understand it well enough to not be surprised by the boar (a fatal mistake in many cases). However, European arts can take advantage of society in different ways. A girl in a village assaulted by boars on a regular basis will literally be brought up from a young age to understand how boars think. They may be taught to garden in specific places, and not others. They may be taught how to run away from a boar, or seek high ground. All of these help a child understand the mind of a boar, long before they pick up a sword.
Accordingly, if you want her to become a swordsgirl fast, make sure the rest of the culture is steeped to train her to understand how a boar thinks. Once that is in place, you'll be able to train her faster than otherwise possible.
