When they changed the stamina at the end of Season 2 (so that lack of minutes had no effect on stamina), it was indicated that the range of minutes for maintaining GS had been broadened a bit. So I think maybe 40-90 will maintain, and a little bit outside won't be too harmful. You probably can't build GS up playing 30 every week, but it might not drop too much. On the other hand if a player is only targetted to getting 30 every week, there's going to be weeks when he gets 20.