My longest commute was 18 miles each way for 18 months. Doable physically, but too much time involved for me (1 hour each way minimum). I did not get any overuse injuries but any little thing would be a big deal with those kinds of miles to do every day. My boss was understanding too. That made it possible. I am not riding through a squall line with lightning snapping all around to get to work. I remember taking two "sick" days due to weather and a handful of late days (had to wait for a gap in the weather). I was really happy to be done with those long commutes after a year and a half.

My optimal commute would be 5-8 miles each way. I might jump out and do 15 to 50 before work on a whim now and then but given bad weather, injury, illness I could opt for the 5 mile route until the situation improved.

I'd say if you are not Superman, and there is some alternate viable option (bus, train, carpool) that you could use when you just "ain't feelin' it" that most people could manage 18 miles each way and be happy about it so long as the rest of your life does not suffer (wife, kids, hobbies, friends, sleep, etc.)