Different schools do it differently, some you'll hear back in a month or so, some you'll hear back in maybe 5-6 months. If you applied to Pitt, for example, they don't even start doing interviews until October, and they keep doing interviews almost into March. Many other schools though, start interviewing as early as August. As far as rejections, if you don't get a rejection in the first month or two, usually you're put into the "maybe" pile, and you're still being considered for an interview.