My full commute is 23 miles each way, gently rolling with 2 good size hills. I don't do it every day; aside from the fact I have to leave work early to get my kids and drive in 2 days a week, even if I didn't have to leave early, I'm not sure I'd want to commute 46 miles every day. When I get home I've got 3 young kids and a wife, so I think I'd be just too tired. To compromize I often drive about halfway and cycle 12 miles in and out.
Now if I were a young buck again weighing 8 stone, full head of hair, racing whippet stance, single and without a care in the world, more full commuting would be on the cards. But I'm not: I'm fat, bald, middle aged with a wife and 3 kids and with a propensity to mutter "that's another house they've built...that wasn't there before!" or stare wistfully out of the window and wonder whether the Morris Ital was uglier than an Austin Allegro (definitely the noddy car Allegro, by the way). No, I wouldn't want to do full commutes everyday of the week.
I agree, a 20 mile round trip is the ideal length for me given the terrain. It would be great to be able to only take 30-40 minutes to cycle in to work.
Personally, a 40 mile round trip would be about the maximum I would be happy with.
A 20 mile round trip would be ideal...not too short or too long imo.