How often you should change your hot tub water really varies depending on how effective you are at maintaining your water with hot tub chemicals.
It is recommended that Lay‑Z‑Spa water is changed regularly, and at least once every 1‑3 months. The frequency of water changes should reflect the degree of use of the hot tub, as well as the results of chemical testing. If chemical levels have fallen far outside the normal ranges, it is recommended that the water is completely replaced and re‑treated.
It is important to shock dose your water on a regular basis and you will find it will extend how long you can use the water for.
Proper chemical maintenance, using the cover, and showering before using your hot tub will prolong the lifespan of the water.