Here's a piece of info I found on a professional pool/hot-tub website, this should solve your question:
"This depends on the size of the spa and the average daily bather load. Water should be checked three time per week for those using chlorine, bromine or a mineral purifier. Also a water sample should be taken to the dealer for analysis one time per month. If your spa cleaner is good then you have to change water only once in a month."
This completely depends on the number people using it and number of times it has been used. If you are using it regularly the you should drain it monthly or if your are using 2-3 time a week then drain it after 2-3 months.
Empty and clean your spa, if you aren't using chemicals, at least once
per week. Although you have a filter on your hot tub, which should
remove most of the dirt and grime, you don't have any chemicals to treat
the water that remains in the hot tub. This means your water will need
to be dumped out and completely changed each week, so you don't have
harmful bacteria building up.
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