My carbon seems to last about two grows. If I leave it in, I start noticing some odor leaking out toward the end of the third grow. And by "grow," I mean from seed to harvest in the same cabinet with the same filter. Your mileage may vary.
Some are under six inches, some are over six feet, no two gardens are the same and everyone grows different plants. They fail when they fail and that's when you swap out the carbon.
Avoid brick and mortar stores. The aquarium stores will charge $5 a lb and more. Ebay sees it regularly at under $2 a lb. Look for a guy named thirdmillenium. Pellets will do, granules are better.
Activating carbon requires firing in the neighborhood of 1500ºF. I wouldn't trust homemade to last very long.
My 24" tall 8" ext dia ones ( DIY from the carbon scrubber for dummies tutorial), they last about 8-12 mo running 24/7. Carbon from the ebay guy. Work flawlessly.
pet stores carry carbon, and of course the net...... kitty litter? really...would think that shit would be to dusty? and used? your joking right?
i suppose depends on a great deal as stated above...
my helpful hint is that once in a while you should shake or re-arrange the carbon as to make sure the peices packed next to each other get exposed also, if that makes sense?