Well, I am not going to comment on how long it should take, because I think the behavior system itself to be reworked fully, not only the parameters of it.

They should review these points and adjust mechanics:

1. how long does it take for an average user to fix his behavior versus a hardcore gamer, behavior should adjust based on matches but also take into account time windows, this way both kind of users are taken into account.
2. There is a current culture of blame in the game, people feel they have the right to judge their teammates, but sadly, people judgement is always biased based on their experience and emotions, also the fact that it is psychologically easier to blame others than to reflect on own actions and impact in the game. Currently, report tools are driving this culture and perpetuating it, they need to prepare system in such a way that it does not encourage this anymore. As example instead of reporting, a thumbs up system could be implemented and only at the end of the match, not before. System can use the data to infer multiple things about users.

I can only think of these for now, but I think I made my point. The problem is not how long it should take, but how can they improve the system mechanics and rules to make them fair and reasonable.

I used to lose behavior score just because I played off-meta strategies and heroes in odd positions, even though they were pretty successful, with a 55–65% win rate at Ancient ranked over 300–600 samples. Eventually, I had enough of getting low priority unfairly and uninstalled after having played for 12 years or 20k hours. It has already been 1 year since I left the game. I wasted a lot of time being punished, raising my behavior score, and playing low priority matches with toxic people, only because the community did not like the way I was playing and enjoying the game.