At 10-12, your freshmen who start at F will get to C/C+, sophs will get to B-/B, JRs to B/B+, and SRs to B+/A-. Give or take a little based on HS GPA and work ethic. 10-12 is probably optimal, as in more than that starts to yield diminishing returns. But everything has diminishing returns, so unless you’re running a combo defense and practicing FB as a late game strategy, you’re probably “wasting” minutes somewhere. It’s all about what you value enough to max potential; ratings, IQ, something else.