It depends on a lot of factors, such as the size of the pool, how often it's used, what kind of chlorine or other chemicals are used, etc. But generally speaking, it's recommended to change the water in a swimming pool every 2-3 weeks.

It depends on a number of factors, such as how much it is used and the type of chlorine used. With chlorine, for every pound per day of use, about ½ gallon per day of fresh water should be added to the pool to maintain correct pH and chlorine levels. So if your pool holds 10,000 gallons of water and it is being used at the rate of 1 pound per day, then 5 gallons (10,000 ÷ 20) of fresh water should be added each day to maintain pH and chlorine levels. If you're using bromine instead of chlorine, then consult your manufacturer's instructions.