If the PH is very low, you don't want to swim. If you are just adjusting the PH to somewhere in the normal range, than 30 minutes with the pump running should do it. But if you are lowering the PH below the normal range for some reason, lowering TA and fresh plaster come to mind, then you don't want to swim until the PH comes back up to normal levels. If in doubt, test the PH after 30 minutes, and if it is in range, 7.2 to 7.8, you can swim.