How often should I change the water?

This is a difficult question to answer, because it depends!
Wooden tubs are unsuitable for chemicals, which means water should be changed every 2-3 days when in regular use, with the lid fitted while not in use to ensure no debris finds its way in.
For lined tubs (fibreglass or polypropylene), chemicals such as chlorine, or filtration systems can be used to maintain water quality for a period running into weeks.
Following basic practices such as showering before using the hot tub can extend the period between cleans.
In a commercial setting (such as glamping sites or holiday rentals), water should be changed for each new guests’ arrival.