I know some colleges offer boxing classes or have full competitive boxing programs. So, one or two semesters should be enough time to build a student up to competitive levels.
I’ll be honest, with hand to hand styles, “highly proficient” is a lot more subjective than you might think, and it makes this question a lot harder than it would seem.
A boxer is going to have to train constantly if they want to make a professional career out of it, and they will always be at a disadvantage when facing able bodied competitors with more experience.
Note the “able bodied” part. As with all sport fighters, injuries do pile up over time, and with boxing, concussions are a real risk, and can impair a more experienced fighter.
With non-sport combatants, more recent training has the advantage. Actual combat styles, including self defense, military and police forms evolve ridiculously fast. To the point that most of my hand to hand training is completely out of date. It will still work against untrained opponents, but I can’t take that and use it against someone with police training. And, again, makes the question of “highly proficient” really hard to answer.
This isn’t an issue with most sport forms, because the rules create a stable structure to work with, and it isn’t an issue with most highly traditional forms, those are trained a specific way because that’s how it’s done, and anything else is just incorrect.
In those cases, “highly proficient” will be a very rigidly defined point, but how long it takes to get there will depend completely on the style and it’s approach to progression.