I break my practice up into smaller pieces than 15 minutes. I give myself 6 hours to get between 45 minutes to 1 1/2 hours of practice daily, and Sunday is a no practice day, however I may play a few pieces (play… not practice). I stretch s-l-o-w-l-y before each session. The main reason I break my time up, though, is not because of the injuries, but rather, so I can stay focused on the (joyful) task and fix the little problems and smooth the difficult passages. It keeps me from being lulled into mindless practice.
I think I even practice a little when I am falling to sleep… At least in my head.