There is no way to know for sure how long it will take, but the more consistently you keep it at shock level, the faster it will go. How high are you taking the chlorine each evening? Test an hour after shocking tonight. Then, test again in the morning within 2 hours of sunup. See how much chlorine you lose. If it is more than 1ppm, then add enough bleach to get it back up to 15 or so. In a 20K gallon pool, each gallon (4 quarts) of 6% bleach will add about 3ppm of chlorine. Run your pump 24/7 and backwash when your filter pressure rises 5-10psi over clean filter pressure.