My commute to work takes about 20 minutes by car and 40 minutes by bike. Going home, I add a few miles to my bike commute, so it takes about 50 minutes. However, the car commute can take much longer on days when traffic is bad or there is an accident. My bike commute always takes about the same amount of time regardless of the traffic, but I am sometimes 5 minutes slower on days when my legs are tired.
When you look at the bigger picture, however, my bike commute saves me a lot of time that I would have spent riding my bike or going to the YMCA after work. Now I get my exercise riding to work and back, so my commute is no longer wasted time. The biggest surprise I had from bike commuting was how much more free time I had in the evenings because I no longer had to squeeze in a workout after work. It's also made things easier on the home front because my wife used to get frustrated with me working out after work rather than wanting to eat right away. Finally, I've also saved $80/month I was paying for the Y membership that I canceled after I started bike commuting.