The common answer voiced by all is every 3 to 4 months. But it actually depends on 2 factors:
First how often you use your spa, and the number of people that use it. The more it’s used, the more chemicals you will add to the water. When the chemicals you add do their job, they don’t disappear, they become Total Dissolved Solids, TDS. And when the TDS is too high, your water becomes over saturated, and any new chemicals you add do nothing.
Think of continuing to add a teaspoon of salt to a cup of water. Each teaspoon raises the salt content, and at a certain point your water becomes over saturated, and the new salt merely settles to the bottom of the cup.
The factor is the volume of water in your spa. The more water, the longer it takes to over saturate your water.
So, if you have a normal 300-gallon spa and you and your spouse use it about twice a week, you will probably be draining it every 3 to 4 months. If that spa is a 1200-gallon swim spa you may not need to drain the water for 6 to 8 months or more.
The best advice is to drain your spa when you just can’t seem to get the water to stay crystal clear and maintain the proper pH, Alkalinity, and Sanitizer levels required for healthy water no matter how many chemicals you put in.