Filters can last from a few weeks to several years. In part, it depends on the materials they were made from, and their designed life span. As a general rule, most cartridge filters average 1 year. We recommend that you chemically clean your spa filters every 30 days to extend filter life. Not just spray them off with a hose, or rinse them off in the sink. A good Filter Cleaner & Degreaser will help remove oils and scum from the filter media. If you do not use the Filter Cleaner & Degreaser, and the pH of the water drops significantly, it can make the oils embed into the filter permanently. This will make your pump(s) work harder, and take longer for the water to clear. It will reduce ripping, extending the life of a filter. Potentially doubling filter life.
As far as when to replace a filter, there are few indicators that may be helpful. To a degree, you can look at the filter, and if it looks nasty even after a good cleaning, it may be time for a new one. Discoloration, folds starting to stick together, calcium or chemical buildup that cannot be dissolved easily, and visible tears are just a few common indicators of needing a new filter. Also, if the water tends to stay cloudy after a long filter cycle and is chemically balanced, this may indicate poor filtration. If your heater keeps shutting off, and pulling the filters out allows it to run after reset – a REALLY good indication of dirty/bad filtration. Lastly, if your pump/motor starts to run a little louder then normal, try pulling the filters. If the noise lessens, it’s a good indicator of needing new filters.
For the last couple, if it gets to the point where the heater is shutting off and/or the pump is making noise, this is a little late. Your heater and/or pump may be damaged from this flow restrictions. Keep up on chemically cleaning your filters, and replace them at least every other year.