Mostly trial and error. Some pools need less than 4 hours per day others need far more. It really depends on how clean YOU want the pool and how long it takes to get the proper amount of chlorine into the pool. But I can tell you with some certainty, that you are running too long right now.
I have a 20k pool and run my pump about 4 hours per day with 2 hours on high and 2 hours on low. The pool is plenty clean for my tastes but others may be a bit more picky.
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