Little Neo, et. al,
I picked up a good philosophy on practicing from my banjo teacher a couple of years ago.
"Practice music, not minutes."
Since then, I never actually apply time constraints to the playing and practicing I do. I just spend as much time as I'm comfortable with on any of the instruments I pick up at a given moment.
I've also found that since I have my instruments on stands or hanging on the wall out in the open, rather than in a case, I play them all more frequently.

And yes, Mbo, when I first started learning the guitar one summer vacation many years ago, I did play until my fingers bled. Then I'd tape the tips with gauze and try again. I've since developed good callouses and seen a therapist about that self-destructive streak! :-)

Sean