80 minutes is actually pretty much the border from good to bad.
You shouldn't have you players play more than 80 minutes per week.
50-70 seems to be the best range.
But this week again, I've seen the big randomness in that. someone with 32 minutes improved to 8, some with 59 even improved 2 levels to 9 and someone with 60 minutes went from 8 to 7.
We have both said a lot of things that you are going to regret.