carbon replacement

[Dorkel Marine 1]

Hello again. I've started useing carbon as I try and stay disciplined in keeping my water parameters steady. How often do change the stuff. How long does it take to saturate. I know in my freshwater tank I used to push and if the bag started to break down I changed it. Anyway just curious. I'm using chemi-pure in one tank and a fairly cheap brand from Big Als. I surround the stuff with foam and change the foam every week. I've noticed and improvement in the waters look. But again when is carbon done;or do you change it early and not take any chances.

Thanks in advance

George