Thanks, that's good to know. I thought because the structure of the pellets breaks down fairly easily that it meant that the carbon itself was degrading. Lol.
More Than You Ever Wanted To Know About Carbon:
Carbon has a finite amount of absorption/adsorption capability no matter the form. It's all about pore size. In addition to having significant amounts of phosphate and a higher ash concentration, cheap carbon will usually have very tiny pores (not enough absorption) or very large pores (not enough adsorption). It doesn't matter what form the carbon is in - at a chemical level this is still the same.
Quality carbon has a good mix of pore sizes, all of which should be in the middle of the scale. It will have lower amounts of phosphate and less ash. Be aware though, most carbon readily available in a retail environment has phosphates. You have to specifically look for steam-activated/phosphate-free, although I'm seeing that more and more these days. The nice part is that soaking the carbon before putting it in your tank will allow much of the ash and phosphates to leech out. If you're concerned about algae, this is an easy way to avoid the phosphate problem (which is really only an issue for marine tanks, but that's another subject).
Carbon gets "full" when the pores are full of pollutants. They're chemically bound to the carbon and so cannot be released into the water. So for all intents and purposes it becomes inert and bacteria start to grow on it. If you don't change it, it's just another form of bio-media. Generally this happens every 3-4 weeks. This varies based on the amount of carbon you're using and your water quality.
One thing to be clear on. Carbon does NOT remove ammonia, nitrites or nitrates, or most heavy metals. At all. It does absorb plenty though - chlorine, dyes, all sorts of alcohols, even bleach. It's great for driftwood as it absorbs tannins (to a degree). And perhaps most importantly in the aquarium, it removes most medicines. If you have to treat your tank, put some fresh carbon in when you're done.
Also, despite what you read on the Internet, you cannot re-use carbon by baking it in the oven. That is, unless you have an oven that goes to 1700°. That's the only way to burn off all the organics.
This has absolutely nothing to do with the OP's issue, but the question was asked and I just couldn't help myself.