I don't know when you get diminishing returns on IQ minutes but I am also sure it doesn't really matter. The more minutes you put into IQ, the less minutes you have for individual skills.
I can tell you this. I have noticed a significant difference when I switch between 15 and 20 minutes of IQ practice. I also have notice that I RARELY have A+ IQ's even when I use 20 minutes for 4 years in a row. I have played teams with 3 and 4 A+'s at least reasonably frequently(but almost always in the tournament), so I think 25 minutes for 4 years will get you to A+.
I think it isn't worth it to go above 25 but if there is diminishing returns it is above that threshold.