All filters need to have their media rinsed clean (in aquarium water) periodically, if only the maintain a good flow of water. Typically this is every 4-8 weeks depending on how messy your tank is.
Carbon is an excellent substrate for biological filtration. After a couple of weeks the cracks on the carbon will be filled with bacteria. While not as good as, say, Siporax, carbon is an above-average media for squeezing in as many bacteria as possible per cubic centimetre.
But here's the thing: once carbon is covered with bacteria, the carbon no longer adsorbs organic chemicals in the water. So chemical filtration stops. That's why you are told to replace the carbon regularly.
Having said this, I recommend against people using carbon at all. It serves no useful function in a well maintained freshwater tank. Water changes will dilute dissolved organics, as well as remove nitrate and phosphate, which carbon doesn't do. Carbon is "old school", a solution to stopping the water from turning yellow back when people did 25% per month water changes. No-one should be doing that anymore, hence carbon is redundant. Moreover, carbon removes beneficial organic chemicals such as fish medications. If you try to treat whitespot (ick) in a tank with fresh carbon installed, you'll notice the medication doesn't work. Incredibly common mistake this.
Most people who use carbon have no idea what it's for, and this is why carbon sells most strongly at the "entry level" end of the market as cartridges for small, relatively inexpensive filters. It's an easy way for manufacturers to make money selling something dirt cheap for them but expensive for you.
Just say no to carbon!
Cheers, Neale
