Not long. Four hours, maybe, with my brother when we first bought the game.
Unfortunately, even in the summer when I don't have seven hours of school plus one hour travelling, my mum insists I eat, sleep, communicate and get some exercise, to the extent of making up a shopping list and sending me down to the shops with it. I can't play for more than 2 hours now before my mum tells me I'll be fat and/or my brother insists he wants to go on.