This next Blob of info is from the Players Guide in the Drop Down Menu under the HELP Tab, up at the top of the page. Generally everyone starts at 20. Myself for individual skills I've never gone over 20, but for Offense and Defense, I bump my minutes up to 22 and leave them there all season, every season. I disagree with Weena in that the point of Diminishing Return for Off/Def sets is 25, not 20. For individual Player Skills, I can't say where the line is.
***One of the crucial areas in understanding player ratings is understanding how to improve them. Solid practice plans can make an enormous difference in a player over the course of 4 years, so here are some helpful tidbits for setting yours. First, abide by the rule of diminishing returns. Spend no more than 20 minutes on an individual skill, or no more than 25 minutes on an offense or defense (apiece, not combined) and you will begin to encounter diminishing returns. There are very few reasons for a coach to break these rules, none of which you are likely to encounter until DI. If a player has maxed out his potential in multiple areas, it may be the right decision to go above 20 in some categories, but these exceptions are very rare.
When setting your team practice (offensive and defensive sets) 20 minutes apiece is considered a fairly standard amount, but this can certainly be tweaked. If you have a very young team with very low IQ's, you may want to bump that number up to 25/25. Conversely, if you have a veteran squad with very high IQ's, you may want to bump it down to 15/15. IQ's improve very quickly when in the low ranges. Your players can get from an F to a C- or so rather quickly. However, as you start to climb the ladder, it gets harder and harder to improve.
For a better idea, take a look at Iguana1's BBIQ Practice Minute Chart, which is a very useful tool.
It's also important to understand the role that a player's potential has on improvement. The rate of improvement in any individual category will be based on a player's playing time, practice time, work ethic and how near/far a player is to reaching their potential. The further a player is from their potential, the larger the initial gains will be and vice versa. When you are recruiting, a recruit's potential will be available to you via scouting trips and/or if you have purchased the Future Stars Scouting Service coverage for that recruit's state. Regardless, you will receive an email from your assistant coach touching on areas of high and low upside for all of your players after exhibition games have begun. In addition, you'll receive notes from your assistant coach throughout the season to help guide your practice planning, e.g. "Coach, seriously, I just don't see Stewey Griffin getting any better at free throw shooting, you may want to think about having him focus on something else like ball handling." ***