I was told by a CM within the last month that they typically allow a fifteen-minute window. That is, they'll let you in up to 15 mins past your FP expiration. However, I'm not sure that's official/formal policy. Generally I find that if you're very nice about it and admit that you're aware that you're late, they'll let you in. Also, if they say no, take it with grace and a good attitude. You can always grab another FP.
FWIW, last Sunday I was about 10 mins late for my BL FP window. The CM was very kind and let me finish my tea at the FP line before going in. In fact, I stopped by Town Hall to make a guest commendation. She was very nice about everything.