Activated carbon only works for about 3 days, then it has done all its going to do and would need to be changed.
Activated carbon is good for a few things, clearing meds out of a tank that has recently been treated, helping to remove tannins from water stained by drift wood or as a preventative measure when airborn chemicals are in the house (think flea bomb). Another advantage to it is to help polish your water just a little more to help make pictures look just a little better when you are planning a photo shoot. It really has NO day to day function.
It won't hurt your fish and I would keep some on hand to use if I need it at all times. I have a large container of it in the basement lying in wait, but there is NON in any of my tanks. It would cost a fortune to replace it every three days. I do have a tank badly stained by drift wood tannins, but weekly 50% water changes solve that problem for about 4 days a week, every week. If it really bothered me, I would do 50% water changed 2x a week.
yeah drift woods do stain water a lot...