Honestly? They can call you off a waitlist any time until like the 3rd day of classes (depending on the schools you're waitlisted at). This makes life really, really hard. I was an alternate last year AND this year, and I dawdled last summer. That's a bad idea.
Personally, I'd make plans as if you were rejected and going to be reapplying - more experience, classes, studying for the GRE, whatever you're going to do to improve your app between now and next fall. That way if you get off a waitlist, it's a pleasant surprise! If not, you're prepared for the next cycle.