If you are using the Vision Sanitizing Cartridge the answer is simple: change your cartridge and water every 6 months.
Otherwise please keep reading.
Draining and refilling your spa at the proper time promotes clean, easy-to-maintain water your family will enjoy every time they soak. When we use our spas we leave behind oils, lotions, perspiration, etc. Some of this can be removed from the water with ozone, spa enzymes and routine “shocking”. However not everything can be removed and some Total Dissolved Solids remain. As they accumulate the water becomes dull, foamy, or develops an odor that cannot be cleared. The questions below help determine when to change the spa water.
Harsh winter weather may prevent you from changing the water according to schedule, so do the best you can. The cleaner you are before using the spa, the easier it is to keep the water clean and actually extend the time between changes.
The answer below will be expressed in months, based on estimated usage. Assume each soak lasts 15-30 minutes per person. Spas are meant to be enjoyed and use is not always the same each time. Give approximate answers and use the result as a general guide.