You should change filter media when it can no longer be used for its particular function. Sponges must capture waste, dirt, and general detritus. Bio-media must retain its surface area for beneficial bacteria to grow on. Chemical media must continue to absorb dissolved particles in the water. Media should be changed when it can no longer function correctly.
Time Frame for media
The amount of time any given media takes to lose its capability to function has many factors. Including but not limited to Bio-load, feeding amounts, water changes/maintenance…etc
There is no given time frame for any one specific media because it is dependent on the aquarium specifics itself. With that being said we will discuss a very general time frame for each type of media.
Mechanical filtration should be cleaned with every filter cleaning. Sponges can be squeezed and cleaned out of any and all detritus that was captured within itself. When cleaning sponges the water will turn a dark color from the release of dirt and detritus that was contained within the sponge. Once a sponge can no longer release the detritus captured it’s time for a replacement.
Course sponges can last a lot longer than a fine sponge. Fine sponges capture so much debris that it will be completely full and cannot be cleaned. Fine sponges should be replaced with every filter cleaning. In general your course and medium sponges can last between 3-6 months before needing replacement.
Chemical media removes dissolved particles from your tank water with activated carbons, resins, and other adsorbents. Once this chemical media is “full” it can not absorb any more and will need to be replaced. In general most chemical media should be replaced monthly or with every filter cleaning.
Some filter media, like bio-media, does not need to be changed. Other media like sponges can be cleaned and re-used multiple times before needing to be changed, usually in about 3-6 months. Chemical filtration definitely should be changed every 1-2 months of continuous use.