I really don't think you go wrong with anywhere between 20-25 minutes for team practice (I sometimes go 30 for defense if I am running zone/press). If you do that and then focus the practice minutes for young players on their cores (i.e. if you have a freshman center with green ratings in rebounding and perimeter, plow minutes into the rebound rating to max it out quicker and then focus on perimeter instead of distributing them equally early on), you'll be fine.
I only revisit my allotments later in the season if I notice that a player is maxed out or very close to it. In my experience, a senior player (that was recruited by me) with 20/25 minutes on O/D always ends up at A/A+ in O/D and yellow/red in every individual skill.