If you were to leave it in place a long time, the stuff it adsorbed would leach back in to the water.
This is not really true at all. This is a boogie man story programmed into our psyche in order to frighten us so that we will continue to remove and buy more product.
The fact of the matter is that there is no physical or chemical reason for the substances that were adsorbed by the carbon to suddenly release their hold.
The only reason to remove the carbon is that the surface becomes filled with adsorbed substances and therefore the carbon can no longer adsorb any more.
If the water still contains toxins, tannins and other pollutants, after a few weeks, then yes, of course, remove the carbon and replace it with a fresh batch in order to continue the cleanup.
The misunderstanding about leaching originates from the fact that some companies choose the acid wash method of activating the charcoal. The acid used is typically phosphoric acid and zinc, so carbon activated in this way may later leach phosphate residues into the water column. This is not a big deal for freshwater tanks but is a consideration in saltwater tanks.
Problems with algae are often caused by the release of phosphate from activated carbon with high POI (Phosphate Output Index) levels - often as high as 14.0, which inhibits lime synthesis in coralline algae, corals and other organisms that deposit lime.
Again, this refers to marine tanks but in planted tanks this is a good thing. Phosphate does not cause algae. Lack of Phosphate causes algae.
I always change it before the said replacement date.
Leaving your carbon in place converts the pellets from a chemical filter media to a biological filter media and is a more effective bio filter media than 90% of products sold as bio-media.
People really need to get over the carbon paranoia. Activated Carbon does 100 times more good things than bad things.
If you are really scared of carbon then just do more frequent large water changes - which accomplishes exactly the same thing as Activated Carbon or Purigen - or of any chemical media.
Cheers,