Good question. I think it depends upon the circumstances surrounding your commute, and is more than just the number of miles involved.

My commute is 36 miles round trip -- but I don't consider it long, for a couple of reasons: (1) it takes me 1:15 each way by bike, compared to an hour each way by car, (2) it's mostly flat, (3) it's relatively uninterrupted (it's mostly MUP), and (4) I don't carry a lot of gear on my bike.