Many customers have contacted me over the past couple years asking, "how do I know when it's time to change my filter?". I have been gaining more and more knowledge about pool filters since launching this site back in January 2010. As a matter of fact, much more than what I knew from my own experiences of being a pool owner. For cartridge filterss, it's pretty straight forward. Almost all cartridge filters have a pressure gauge on them. Typically, a good standard is to note the pressure reading at the time of putting in a brand new filter. In most cases, that pressure reading will be around 10-12 PSI. As the cartridge is used and collects debris the pressure will build up. A good rule of thumb is when the pressure reaches 18-20 PSI it is time to clean them off (hose off with water). Normally, this should be every 2-4 weeks. After placing them back in the filter you should see the pressure drop back down to the 10-12 PSI range. You can repeat this process for months and even years. Your own unique situation will dictate when the time comes that you rinse off the filter and the pressure doesn't drop down much and it becomes necessary to clean the filter much more frequently. When this happens, it's time for a new filter. Another option would be to do a thorough pool filter cartridge cleaning as defined by the Center for Disease Control.
For D.E. filters the most straight forward and leading indicator is the fact that you begin to see the D.E. material finding it's way into your pool. This indicates that your D.E. grids have a tear in them and need to be replaced.