When do you have to drain the spa ?
You have to drain the spa when it has a high level (> 3 000 ppm) of TDS (total dissolved solids) or combined to a high level of chlorine.
Here is the formula :
(Number of gallons / 3) / (Number of swimmers / 24 h) = days of continuous operation
Here is an example of a spa of 8 feets of diameter; 3 feet on depth; average of 4 swimmers/24 hours :
(1 128 / 3) / (4) = 94 days (3 months)