When I had a longer commute (16 - 20 miles one way depending on route), I did it three days a week. I was living alone much of that time, but had housework, yard work, pets to take care of, and I like to sleep... so I didn't do it five days a week.

Now that I have a 3.5 mile one-way commute, I ride to work everyday. I'm living the car-free lifestyle and will be selling my truck very soon (and buying a Big Dummy).