When the topic comes up over in Commuting Forum, it seems that daily commuters tend to peter out after around an hour each way. Beyond that is doable, but due to time and fatigue constraints, most people drop off to 2-3 times a week.
With a four-day workweek, I do a hour each way most days in the three-seasons. That's my long loop, 17 miles.
Through the winter, enjoyment falls off for me with the temperature and snow depth. In the dead of winter 20-25 minutes each way is about all I want to do. Just enough to keep with the recommendations for aerobic exercise. Fortunately, I live only 4.5 miles from work, which works out.