Completely depends on the school. Some could be admitted from the waitlist at any time. Other schools will pool all the waitlisted candidates and review them again in March (or later) if there are still seats available. You never know unless you ask the admissions staff if they are willing to tell you.
It is very important that if you don't see yourself going to a specific school that you turn down any offers or waitlist spots. You may also get a check-in email to see if you still want to remain in the alternate pool (every month or so).