With MOST schools, they will notify the school you're holding an acceptance at when they make an offer to you, and you have 2 weeks to make a decision to switch or stay with your original choice.  Some schools give you a little more or less time.  It also depends on when you're accepted (you won't get two weeks if you're accepted 4 days before orientation begins).  Check with the school you're holding and the school with the waitlist.