Minutes of practice, work ethic and high school gpa contribute to how quickly players learn the O/D sets. My experience has been that work ethic seems more important than the HS gpa. I haven't noticed a correlation with playing time, as my redshirt freshman seem to have the same IQs as players who are starting / getting 10+ minutes as freshman.
I think the general feeling is that there are diminishing returns when practicing over ~20 minutes, but it could start sooner. Maybe someone else will chime in. That isn't to say that practicing 23 minutes or 25 minutes isn't worth it in some cases.
Most freshman will get to a B- by the end of the first year with 20-25 minutes of practice. By the end of the 2nd year, most players will be ~ B+, although if you have really high WE you might be able to get to an A-, and with really poor work ethic it might only be a B. It usually takes the last 2 seasons to get to an A/A+. Some players never get to an A+, especially if they have poor work ethic.
If you are switching to a new set, the freshman and seniors should learn the new set at roughly the same rate, with the main difference being their WE/hs GPA, ie both freshman and seniors would be about around a B- by the end of that first year of practice.