Filter manufacturers will all tell you to replace the filter media/cartridges often, but in reality this is pointless. I will explain.
There are three types of possible filtration...mechanical, biological and chemical. Mechanical is pulling the water through media that remove suspended particulate matter, so the water is "clear." Clear and clean are too very different things. Biological filtration involves the establishment of nitrifying bacteria and this will occur on any surface within the filter and in the aquarium for that matter, especially in dark places. This filtration handles the ammonia to nitrite to nitrate. Chemical filtration involves using some chemical or similar substance such as activated carbon to remove substances from the water, substances such as medications, some gasses, dissolved organic carbons, etc.
Chemical media like activated carbon can become clogged over time and no longer do what they are intended to do. Most of us do not bother with chemical filtration unless there is a serious issue, but this is the only media that needs more frequent replacement.
Provided the media/cartridges are still relatively intact and the water is forced through them and not getting around them, they are OK. Rinse them out regularly. Filters easy to access like internal, hang-on-back, sponge filters, etc, can be rinsed at every weekly waterchange. Large external filters like canisters less often, depending upon the aquarium biology.
That dark brown sludge is organics, and this you want to remove regularly. The organics in the substrate provide beneficial plant nutrients, and are less likely to build up to problem levels. The organics in filter media is not doing any good so keep it clean.
Initially it is safest to use a bucket of tank water to rinse the filter media in. Once the tank is established (a few months) this is not needed, and you can rinse it under the tap.
