i would think it would depend on what you are cleaning with. if you use all natural products like water and vinegar or water and baking soda or borateem i'd think you could do it as often as you like. if you are using the detergents made for steam cleaning machines it kind of leaves a film of soap on the carpet (which attracts dirt so you need to shampoo sooner)
i've had good luck just taking a damp mop to the carpet in between shampoos.
i think it would also depend on how often you vacuum and how often the kids spill etc. good luck
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