Freddie- It depends, but typically a law school will accept a certain number of students to yield the amount they want in their first year class. For example, school X might initially accept 500 students hoping to get 250 that will actually come to their school. If 250 is their goal, and they yield less than this, they will then turn to their waitlisted students. This could happen early, or a few weeks before classes begin. A friend of mine was accepted the first week of class after another student paid their deposit, but didn't show up.
Hence, it is a way for law schools to cover their perverbial butt. If, however, you have been waitlisted to a school you are interested in, you should write them a letter that lets them know that they are your first choice. This is because a number of schools will put their waitlisted students into tiers.
Some schools go to their waitlist every year, others hardly ever.
Hope this helps...