It is true that carbon loses its effect as a chemical filtration method within a couple weeks... if that. Most people at this point would consider it "useless", and it is... as a chemical filtration process. By this time however in an established tank, it is providing for bio-logical filtration.
Though many people will say "don't use it its worthless!" on this board, I couldn't disagree more. I put in carbon to help polish the water, and then I leave it in there, even after its "chemical filtration" properties are worthless. Every few months, Ill change out the carbon media and polish the water again as well as provide some chemical filtration.
This is obviously just a preference, and by no means to I swear by it, but I do think that all three methods of filtration are vital to aquariums.
1. Mechanical - Filter sponges
2. Biological - Bio-wheel, Bio-balls, OLD CARBON
3. Chemical - Activated Carbon
I feel that lacking in any of these forms of filtration would not be wise, but then again... that is just my preference.
-Jake