Although many people will say what I am about to say is wrong, this is what happened to us:
In April of 2007, we got a FP for Buzz. When our time rolled around, there was hardly any lines at all in the park (it was raining and the 1/2 the park closed early for a private party). The CM for Buzz told me to use it the next day. I told him that was our last day in the park, and he told me, then save it for your next trip. We did just that, so in October of 2007, we used them, and no blink of any CM eyes.