If you are going through a reputable Sub program, the entire process, from your induction dose, to the end of your taper, a typical time frame could be 6-12 months.
I wouldn't recommend trying to use Subs on your own to come off Percocets. Unless you're taking a HUGE amount, the Sub is actually much stronger, with a much longer half life. Subs are not a magical pill to be able to avoid w/d's. When a person decides to use sub, it should be as just ONE tool in their recovery.
Sub will not completely eliminate w/d's...you'll have to face them, at some point anyway, so you would probably be better off just coming off the Percocets.
How much are you taking, and for how long?
For people who really can answer you. You will need to be more specific. Time frames and dosages will help when that person comes round.