My commute is 7.2 miles (11.6 km) each way. I try to ride 2 or 3 x's a week.
I live in the 'burbs around a city and there are 3 major road crossings with which I must contend, regardless of the route I choose: Fortunately, I can do them all at traffic lights.
Most of the other bike commuters I know in my office have rides that are actually shorter than mine. Some as short as a mile, if they take the most direct route. When I was in Massachusetts, I knew a guy who commute 4 days/week for 23 miles each way! He was an animal!