Mine used to be 12.8 miles each way. I moved a while ago and it is now 9.7 to work and about 10.5 from work (I take a couple of back roads with a lot less traffic on the way home).

I did the 12.8 miles for a couple of years, and some days in the winter I was tired -- but that was mostly due to not getting enough sleep prior. Now my routine is a lot more manageable, but like others have stated, 5-6 miles would be ideal -- not too short, not too long. I get a lot of podcast listening done when I ride -- about 15 hours a week, so I find my commute to be very productive.