It depends on the catering hall. I think 72 hours is the norm.
If I were you, I'd go and enjoy. But being fired from the wedding party for such an arbitrary reason is not cool. I'd take that into account when buying a gift.
It's a good idea, when you give your guarantee, to give a figure that is about 90% of the number of people you are actually expecting. You will be expected to pay for the number you guarantee or the number who attend, whichever is greater. So if you guarantee 100 and only 94 show up, you're stuck paying for 100; if you guarantee 90 and 94 show up, you will pay for the 94. Catering halls always leave a margin of error, so the "extra" 4 people will not be getting cheese sandwiches or anything.
I gave a 90% guarantee for my wedding, and a few of my guests had family emergencies, so my guarantee wound up being right on.