I commuted at various distances from 8 miles each way (too short) to 29 miles each way (too long). My favourite was a 17-mile each way which took just over the hour. Long enough for a workout, and to do training routines of varying intensities, but not so long as to be too time-consuming. And I found two hours per day on the bike to be pretty much ideal for keeping me fit.

Of course, if one lives closer to work one can always throw in a loop in order to get a longer ride. There's no law that says you have to take the most direct route.