I used to have a 12 mile commute (each way) and it felt too short in the summer, but "a little long, yet doable" in the winter. I bought a house last year and now my commute is 22 miles each way. 22 is fun from April to October, and I'll even stretch it to 30 or more when the weather is really great. However, 22 is not so great November to March especially when it's snowing or raining. Fortunately, I have the option of taking a train, which cuts the riding down to just 10 miles each way. So, I take the train most days from November to March.

If I didn't have the option of taking a train or bus with my bike, then I would say 10-15 miles each way is the perfect distance (for me) for year round commuting. 15 might seem too far in December when it's cold, wet, and dark but it'll seem nice 7-8 months of the year.