We typically recommend a fresh fill every 3-4 months, depending on usage. We can also test the Total Dissolved Solids of your water (ideal range is less than 2000 ppm), if greater than 2000ppm it is time to fresh fill.
It is also a good idea to fresh fill prior to the big winter freeze! This prevents having to do so during the coldest months. When refilling your tub remember to fill by placing the garden hose down into the filter cartridge to prevent any air locks.
If you have specific questions about how to drain and fresh fill your tub give us a call we are always happy to assist!