In New York City, Fall seems to start in mid September. It suddenly gets chilly and then shortly after, the leaves start to change color (that part hasn't started yet). Then all the leaves fall off and it gets even colder, and then I start missing the hot hot summers!
In general, the entire Fall season seems to last anywhere from 3 weeks to 8 weeks. Who knows though lately, as it seems like every year is dramatically different with the weather.
How many weeks does Fall last where you live?