Activated carbon is considered chemical filtration. Activated carbon removes organic polluntants which discolour the water with a yellowish tint, organic acids, proteins, hormones, antibiotic components, and organic compounds as well as a a variety of chemicals, medications, metals and minerals.
Inorganic substances removed are chlorine, chloramine, colours and odor causing substances such as phenols.
Removed organic compounds will still remain biologically active, still decay and remain part of the aquatic system. Activated carbon is therefore best used on a short term basis and replaced regulary. Weekly water changes of 50% are still highly recommended on any aquarium regardless of size or stocking level.
3 tablespoons of carbon per 50g and you can remove it after 24-48 hrs as it is no longer effective. There is no harm in leaving it in though...
With all that being said carbon is essential for a reef aqurium and fish only saltwater aquariums.
It is NOT even nescessary in a freshwater aquarium.
Use it or not. It is up to you. Take it out...leave it in...again up to you. I do not use activated carbon unless I have recently medicated. My well water is high in phosphates. Activated carbon itself releases phosphates into the water column.
So it is up to you my friend. I guess if I were still using it I would change it once a month when I do my filter maintenace.
Hope this helps. Good luck.

P.s. Remember that no amount of filtration or activated carbon will make up for not doing regular weekly water changes. I always recommend 50% on every tank. Especially goldfish. "freshwater fish like freshwater"