Water changes are individual. It depends on the frequency of use, the number of people, the ambient temperature, the quality of the water used, etc. To keep the water fresh and clear for as long as possible, the water quality should be checked regularly with test strips. All values (pH, hardness, chlorine, alkalinity - calcium content) should be within ideal parameters. If they are not, consult your pool chemical dealer and adjust the water. If your hot tub has a saltwater system, this converts salt to chlorine gas. So the saltwater system only addresses the amount of chlorine in the water. It does not affect the other parameters (pH, hardness, calcium).