It depends. Anywhere from 12-48 hours with 24 hours being the norm. Longer for low cell count / older yeast and shorter for larger cell count / fresh healthy yeast. You'll know when it's done by color change and smell. Sometimes you can see the krausen rise and fall but not always. It will look more milky and smell nice and yeasty when it's done.
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Stick with the plan....not the sparge.