The answer to this depends on how often you use your spa, how well you maintain it and what chemical system you use.
As a rule you should change the water in a spas containing 700 to 4000 litres on average every 3-4 months for average usage and every 2-3 months for heavy usage. An average user would use their spa 2-3 times a week and a heavy user would use their spa 4-5 days per week.
Swim Spas containing 5000 to 7000 litres will need to have the water changed on average every 8-12 months for an average user and every 6-7 months for heavy usage.
NOTE:For instructions on how to drain your spa please review the video below.