You want at least 48 minutes for training purposes and to maintain at least a respectable GS. The ideal amount (for strong/proficient GS) in my opinion is between 50-65 minutes. Other people have different numbers but these numbers work for me. You also have to take into account, the random factor. For instance, last week I had a player with 48 minutes who went up to proficient GS. As long as you aren't below 48 mins or way above (like 80+) you should be able to maintain a respectable GS. Respectable GS being the minimum that I like to have my players at.