I used to walk to school, 20-25 minutes twice a day, and did the same time last year at uni.
Now I live about 35 minutes away from uni, and to be honest, I think it does get a bit too much after a whole week. But maybe that's because I don't enjoy half of the journey, being on horribly, loud roads. If the walk was scenic, then I think I would be okay going up to 45 minutes.