Tetra claim their AC
for the external canister filters lasts around a month. I doubt they'd say as long as that if it werent true, since if it were a week they'd be selling more and making more money.
It depends on the brand, I guess.
I've left my AC
running for months and months in two of my tanks without any adverse effect (it's laziness on my part, I can't be bothered changing it out of the canisters). I've read that it's yet another 'aquarium myth' that AC
leeches back into the water once it's 'used up'. In fact, it's inert at that point.