Well I can tell you guys right now I believe at least 65% of the ~185 spots are 'filled', so to speak, with accepted students. They will definitely accept more people until they've accepted ~186 students before May 15. Between now and May 15, some students will withdraw because they will be accepted to a school they'd rather attend. But I doubt that would be a very significant chunk, because a lot of people just hold onto their acceptances (rather greedily, in some cases) for various reasons until May 15.
Basically, once students are forced to declare their single acceptance on May 15, many will withdraw from Keck, and then a lot of people will probably move off the waitlist sometime soon after May 15. That is my understanding, please somebody correct me if I'm wrong.
This is pretty much universally true for any school as competitive as USC (or any other CA school, really). I've read that schools can begin to see where you've been accepted to on March 15.
So to answer your question guannaberry, a small number of students will probably be accepted off the waitlist before May 15, but I'm betting the vast majority of people will move off the waitlist after May 15, and then things will slow down a lot after that.