The frequency of changing water in your cold plunge depends on several factors including the level of use, whether it's for personal or community use, the presence of a filtration system, and how well the water is being monitored.
Remember cleansiness is next to godliness! If you make it a good habit of getting into your plunge "clean" you will inevitably prolong the need for water changes.
Also keep in mind perfumes, lotions, deodorants, clothing detergents, softners are all chemicals that play apart in the embalance of water chemistry.
For personal use, if the cold plunge is equipped with a good filtration system and properly maintained by cleaning out the sponge filter, as well as activating ozone sanitation after each use the water may only need to be completely changed every couple of months or longer. However, it's essential to regularly remove debris from the water to ensure extended use, as well as adjusting the maintenance schedule as needed.
For communal use, such as in a gym or spa setting, the water should be changed more frequently due to the higher load of users, which could introduce more unknown contaminants. This might mean changing the water as often as weekly or bi-weekly, in addition to daily water quality testing and maintenance.
If the plunge does not have a filtration system, water changes should be done more frequently to ensure cleanliness, which could be as often as weekly or monthly depending on the circumstances.
Refraining from adding chemicals is highly recommended not only for the safety of the users health, as well as a good practice in prolonging the life and longevity of the plunge.