i would agree with iguana, that the diminishing returns kick in around 10 minutes. i really think 2 seasons at 10 minutes would yield better iqs than 1 at 20 and 1 at 0. however, i also agree that 20 mins is a bare minimum. i personally go 25, and at times have done 30, but in this new engine, improvement is slowed severely enough that i don't see myself going above 25 unless a significant change is made.
i think the difference between 20 and 25 is noticeably more than 25 and 30, so i would guess the sharp diminishing returns start to kick in after 25 minutes.