I don't really have much of an answer to your question because I'm still a n00b, but I thought the "laws of diminishing returns" goes into effect with >25 minutes of practice in a skill area. I'm fairly certain that applies to time spent on IQ/offense-defense, too.
Generally, for individual practice minutes, I like to spend 20-25 minutes on skills that are still in the blue, 10-15 minutes on skills that are still in the black, and 5 minutes or less on skills that are in the red. This varies from player to player, of course, because some guys have a lot more potential left than others. If I have a guy that's in the red in every category, I just give that player a ton of minutes working on conditioning.
As for IQ, I go with 20-25 minutes. This season, for example, I've got my practices set so my guys spend 25 minutes on both defense and offense. Last season, it was 25 for offense and 20 for defense. I never go over 25 minutes for either category because of diminishing returns.
Obviously every player has different WEs, but you mentioned 10 points improvement in a skill from 15 minutes of practice; by that math, 25 minutes of practice would give you roughly 16 or 17 points of improvement in said skill. If a player has a high WE, then those 25 minutes of practice may very well lead to 20 points of improvement.
That's just my two cents. Hopefully it helps somewhat, but like I said, I'm still pretty new at this game and I'm still figuring things out.