takes me 20 minutes to ride to school.

As for trainning rides, i've done high intensity rides as short as 25 minutes. The only reason was because i was crunched for time. As long as you stretch afterwards it's better then just sitting on your ass watching the simpson's.

Normally 45 mins is the bare minimum for a recovery ride on the rollers, 30mins or less if i'm sick or recovering from injuries. Out on the road 1 hour & 1/2 is usualy what i do as a minimum ride