How about never? It isn't necessary to drain and refill. Most people never do. The only real reason to do so is if you have major problems that you can't clean up, but typically most problems can be cleared. If you maintain your chemistry properly, you shouldn't have to drain. I've never drained --- unless I was installing a new liner.
Question -- what form of chlorine do you use and what is the calcium hardness reading of your pool's water and also your fill water?
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