I'm doing a 28mi round trip commute. It's a workout on the foul-weather bike; on my roadbike, it's a joyride in the morning and after work. I am finding it tough to do all 5 days of the week though, esp. with the Oregon weather adding extra prep time to pull on warm clothes and plastic bags on my head and other such necessities.

Ultimately it'll be whatever you're comfortable with. I'm going to have to figure out a way to make the friday ride easy if I plan on doing 5/wk, as group rides in the area are all on Saturday.