you can get banned one more time it'll be for 3 weeks...after that it's pemanently
There have been reports of people being banned for a month or two.
It all depends on the situation. You could be banned for 1 month then perma banned. or you could if it is bad enough be perma banned.
It sounds like it is the leave buster banning you and not riots people doing it. So if you know your internet is ****ty find out what time of day it isnt ****ty and play then.
Or spend the money you would be spending on riot points to get better internet.