There's no set number - there's a random factor involved.
indeed, I have a player with strong game shape this week, that only played 28 minutes last week, and have players with around 48 minutes in rspectable shape.
I also have some 9-game shaped players, and none played more than 60 minutes. Actually I have some every week, and mostly I try to get them between 40 and 55 minutes of play-time, which works out nice.
I think that if a player gets 48 minutes, he has as much chance of getting the highest game shape, as someone who plays 60 minutes... More minutes might even lower it a little again (not sure though), and when he gets too many (is it 80 or 90 minutes?), he definatly drops in shape...
conclusion and answer to the question: 48 minutes should be enough. (however I think even 45, and maybe a little less, will do...)
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