Even if you’re keeping your hot tub running properly, cleaning regularly, and balancing your water - you still need to change the water periodically. It is recommended to change the water at least every few months, but really - the frequency will vary based on a number of factors.
Frequency of use, number of users, environmental factors, spilling beer in the tub, and other random mishaps will all impact your water and the frequency with which you will have to drain and refill the tub.
There’s even a formula to help you figure out when to change your water. The formula is pretty simple - it’s hot tub gallons divided by 3 divided by the number of users per day. The result is the number of days between water changes.
So - if you have a 500 gallon tub and 2 people using it daily, it would be 500/3 = 167. 167/2 = 84. You should replace your water every 84 days.
Regardless of the math, there are other times when it’s a good idea to change the water. If your water smells funny, if it is cloudy and shocking it doesn’t clear it up, if there is a lot of scum floating on the water, it is probably time to change the water.