There are a number of variables that affect the life of your spa filter such as the amount your spa is used, and how the filter is maintained. Over a period of time your cartridge will gradually block up and will need replacing.
Pleated filters should be cleaned at least once a weeks.
Micron filters cannot be cleaned but need to be replaced every 3-6 months if the spa pool is being used regularly.
Some signs that your spa filter needs to be replaced are:
1. The spa water is not staying clear. 2. The cartridge gets blocked quickly, and you have to clean your cartridge more often. 3. Restricted water flow and high differential pressure (differential pressure is the difference between the pressure coming in and the pressure going out, caused by a blocked cartridge). 4. A cracked or damaged cartridge, which is very rare unless the filter has been plugged or there is an issue with the spa plumbing.