Depends a lot on school and field and the habits of the director of graduate admissions.
It would also be sensible to e-mail the director of graduate admissions at the institution where you are waitlisted and ask them if they typically admit students from the waitlist, and/or when you are likely to hear back (this is early April for most places). You might also ask if the waitlist is ranked and if so what place you are on it. Obvously, top people on the waitlist have a much higher chance than someone further down.
Also of course check the acepted students page on this website for previous yrs waitlist reults