I use every single one of mine, every week. At first I just tested it out, to see how many I could use without gaining (taking into account my activity level and whatnot.) I discovered that I am one of the (very) lucky few who can use up all the weeklies and still lose.
I feel awesome about being able to use my weeklies, because I refuse to give up any of the foods I love. I just...won't do it. I don't see the point. So I really feel for people who have to give things up for various reasons.