The answer to this depends on each school. Most schools have rolling admissions but some schools do not. And the response time post interview is different for each school. Some dates to remember are:
October 15th–The first day that acceptances can be sent out
March 30th-The date when all MD schools are recommended to accept at least the number of students that will be in the upcoming year’s class
May 15th-The date when all applicants must choose one school out of multiple acceptances. This is when most of the waitlist movement begins.