I understand that you need to take a bus to Geneva to flay back to the United States. Is this correct? How many days show I allow to leave Chamonix and get my flight from Geneva?
Chamonix is not far from Geneva but If it were me , I'd leave a day . You could go to Geneva the day before your flight and spend the last night there , I am not in favor of making that trip , or any other , the day of the flight . There is also rail service from Chamonix to Geneva .
My basic rule is that there has to be two modes to get to an aiport from where I am. One has to be mass transit (bus/train) the other has to be individual transportation (car/taxi). I've ignored it for a late-in-the-day departue and driven in from a couple hundred miles out, but knew I could be left blowing in the wind in the process. I've also been down to Plan C from twenty miles away.
You can reach Geneve airport by train from Chamonix. Paris is also not that far.
There is a very frequent mini-bus that departs Chamonix for the Geneva airport every 45 minutes. http://www.chamexpress.com/