well, oldresorter wrote the players guide, and his advice should not be taken lightly.
however, in fact, iguana once demonstrated that diminishing returns kick in IMMEDIATELY for team planning, and honestly, i suspect the same to be true with individual planning, at least to an extent.
really, the issue is this - the use of "diminishing returns". really, the phrase means, subsequent effort yields lower returns than previous effort - and in that sense - you hit diminishing returns right from the get-go - you get more out of the first 5 minutes of team planning than the next 5. however, those next 5 are still quite useful. so what i think you are really asking is, "when do diminishing returns kick in to a serious extent where i start to really waste them". anyway, maybe i am going overboard here, but i think that is an important distinction. you definitely get more from individual minutes, going from 10 to 15, than 15 to 20. i dont want to say the line is 20 minutes and have you think every minute from 1 to 20 is equally good - its just not that way.
anyway, the general consensus for when diminishing returns REALLY kick in, if you will, is 20 minutes on individual skills, and roughly 20 or 25 on team practice. i actually often question if they follow the same curve. my opinion is that in both cases, 20 to 25 minutes is still pretty significant (but if you had 2 really important categories, its definitely better to balance - because simple diminishing returns are in effect the whole way). 25 to 30 minutes is noticeable but only by a keen eye over the course of a season. and beyond 30 i think you are almost throwing them away. hopefully that helps... and hopefully i didn't too over complicate the issue and make it worse.
for a reference point, i start freshman bigs with low sta and high sta potential at 24 or 26 conditioning literally all the time, and go as high as 28 on a fairly regular basis. this is freshman, not maxed guys with nothing else to practice. playing the press, stamina is vital, and when you have a guy in the upper 50s or 60s, he just really struggles. so i think its well worth the extra minutes. generally speaking, i don't have any freshman above 20 in anything but conditioning, except when they are those weird freshman who have high starting ratings, and like 4 key high potentials and low everything else.