kept at room temp should be good for about a week, that is if it lasts that long. Thats at a cool room temp. if its really hot where you are then you might want to throw that cake in the fridge
just don't touch the fondant when you take the cake out of the fridge, because you will leave finger prints with the condensation. Other then that i haven't had any problems refridgerating a fondant covered cake.
