Are you asking when to hear back about an interview? It can really depend on the school, but it's a rolling process. Some schools get back to applicants within a week, but others can take a month or more. If you're interested in a specific school, you can look at their school-specific thread for answers, or call the admissions department.
If you're asking when people get accepted from a waitlist, it's also rolling. It tends to pick up in mid-May, when applicants have to commit to just one school, and therefore turn down other acceptances and waitlist spots.