Zero tolerance (ban on sight) has no set time period of which you stay on the list. You stay there until something (meaning: the staff thinks you've stopped being a complete dimwit) happens, and you make some form of appeal.
If you have ban appeal access, you could write a lengthy appeal after several months of being a semi-pleasant forum user. If you don't, you could possibly try another contact methods such as IRC and messaging an administrator. Typically if you're genuine and don't sound like a whining five year old, they'll listen to what you have to say if you're around.
TL;DR: BOS has no set time-frame of which you set on the list unless specified; typically you don't leave unless you've stopped being an idiot and the staff notices that.
(The policies might've changed in the past year; I'm writing this solely based on my knowledge as a former staff member. My apologies if there's some misinformation in here.)