Some of that depends on how quickly you pick a play and hike the ball at line of scrimmage. Some people use 15 mins with a 12 second clock runoff. Others use 12 or 13 minute quarters with a 15 or 18 second run off. You want to pick a quarter length and runoff that will give you 120-140 total plays in the game - runs, passes, and sacks.
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