There is no hard and fast calculation that I know of. Even beyond the practice minutes, factors like work ethic rating, high school GPA and minutes per game also impact how quickly a player will improve. If you have a guy with a low work ethic and low HS GPA who only plays 7 minutes per game, he may never maximize his potential, even by the time he is a senior.
The general consensus is that anything over 20 minutes on a skill provides diminishing returns (i.e. the difference between 21 and 20 is much smaller than the difference between 11 and 10). It also depends on what position the player plays and what skills he has maxed out on. For example, it is easier to pour skill minutes into a center that has a low perimeter and low ball handling rating (because you can set both at zero without harming the player) than it is with a small forward who is pretty good at everything. But in general, I would say between 15 and 20 minutes should help you maximize potential as best you can. The only time I go above 20 is when a player is a senior and has maxed out enough of his skills that I need to put those additional minutes somewhere.
And for team practice, anything more than 25 minutes is overkill. If you have a young team that doesn't know the offense/defense, put 25 minutes into each. Once your key players get to B/B- range, drop it to 20.