20-30 minutes is often listed as a good target for daily exersise. If you are worried about health benefits, find a 20 minute route.
Other options that I have used: Buy a fixed geared bike for commuting, and do interval training on the commute. Fatter tires and a heavier bike can also give you more of a work out. But the most important thing is that you enjoy the ride, whatever you do (while keeping it safe).