In my humble opinion there are no "better" or "worse" minutes within the desired boundaries (48-72 min). Stamina does not matter when players are in that range. I.e. it is irrelevant whether your player has 9 ST and you gave him 48 min or he has 1 ST and you gave him 72 min. As long he is in the range he should generally improve (maintain) his current GS with a lot of randomness and backtracking included in the equation ofc.
Here are three players of mine, with different ST levels, who (with very rare exceptions) only get 48 min each week:
(49611812),
(49567405),
(49554209). Still their GS is consistently (close to) perfect.
After the changes stamina comes into play when players are over 72 min. The higher their ST, the more minutes over 72 they can get without suffering GS penalties. We are talking about 7-8 min max probably, even at 9 ST.
Massage indeed allows their GS to withstand several extra minutes weekly as well. Psychologist helps with problematic GS/minutes too.
BBB: 2 (S37 S38); Top tier: 7 (S35 S36 S37 S38 S39 S41 S63); Cup: 9 (S25 S35 S36 S37 S38 S39 S40 S41 S61)