the minutes told by king are the perfect minutes to maintain GS.but if you would ask me anything between 48-75 minutes will keep the GS of your player at strong-proficient.
just remember one thing.while calculating the GS of a player,the minutes of the last week as well the week before that(and to a much lesser extent the week before that) are taken in consideration. so if your player plays like 70+ minutes in one week dont play him over 55 minutes in the next week so that the average of both the weeks is close to 60.i hope you get my point