How long is a 'season', in terms of years, for American TV shows?
For the large networks, such as ABC, NBC, CBS, FOX, a season usually begins in September and ends the following May. The number of new episodes varies - you'll see most of the new shows in "sweeps" months when Nielsen compiles the ratings. And plenty of reruns during the Thanksgiving & Christmas holidays.
Cable shows vary even more from the above; some start in the summer and go into the following year based on how many episodes there are; usually about 12-15 per 'season'.
Hope that helps!