Thats why the countrys will select the quantity of minutes per divison in a season
48 min x 30 games = 1440 minutes in a season.
Could be:
400 for 1st div.
300 for 2nd div.
600 for 3rd div.
700 for 4div.
Thats use 1 U21 player for 13 minutes (+/-) in each game and i'm not counting Cup