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 spas containing 700 to 4000 litres on average every 3-4 months for an average user and sooner for heavy usage.
An average user would use their spa 2-3 times a week. 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 average usage and every 6 - 7 months for heavy usage.