I generally do about 12 each way, and find it quite manageable even at my slow (12.5-13.5 MPH avg) pace - even as a fat guy (I'm not technically a Clyde - just fat). I find doing the entire 20 mile distance each way from home to work a bit much, but that's largely because of time (what I ain't got).

BUT as a caveat, I also have the option of a very short (1.5 mile) ride to metro, a longer (but still under 12 miles) ride to metro, a wife to take me to metro, etc... so I am not bound to cycling every day, no matter what. Presuming you have alternatives if needed, I'd say 15 each way should be no problem.