I'm traveling in November. My return flight is 1:35 in the afternoon in Florence, but I will be in Siena the night before. Should I try traveling a day earlier to get back to Florence, spend the night there and then catch my flight that afternoon or am I allowing plenty of time to get back to Florence and to the airport if I leave that morning from Siena?
You should be just fine. Amerigo Vespucci airport is quite small. Plus, I'm betting that this is an EU flight as I don't think that there are international flights to the U.S. out of Florence. There is a SITA bus that can get you from Siena to Florence in about 70 minutes. (Take the Rapido express bus). Then, I think that you can take a SITA bus from the bus station to the airport. If you can get to the airport by 11:30, you should be fine.