Every couple weeks is a good top end.. the carbon is usually done adsorbing the junk by then.
I use black diamond or whatever decent brand is on sale.
I have never seen any realtime evidence of carbon "releasing" OC's back into the water after it is "done" adsorbing waste compounds (which it does partially by a chemical bonding of the waste to itself and partially by mechanical filtering) and sometimes I have left a can of carbon in my magnum 350 running for months because I had traded out a bunch of guppies and the bioload got low for awhile. The water stayed fine for me and I believe I had just regular mechanical filtration for that latter time after the carbon's adsorption properties were done for.
Reefrunner69 changes his GAC
out once a week in his nano which would be a really safe bet if you dont mind spending an extra buck or 2 a month (which to me is worth it if you have high dollar corals and stuff