Here's a direct quote I got in an email from one of the people who interviewed me at Tulane regarding movement from the waitlist: "This will start after the first of the year and then after 15 May is a landslide."
Obviously it's only one guy from one school, but I hope it helps you. After May 15 we all have to narrow our choice to one school, so the bulk of the movement occurs after that. Good luck.