I mostly just eyeball the amount.
The tanks that I mainly use it on both use replacable carbon filter cartridges that have a plastic backing with slits in it.
I just cut along the edge of the plastic backing leaving one side uncut and then peel the plastic backing back.
After that I just dump out the used carbon and then refill the cartridge with new carbon.
I put a couple of rubber bands around the cartridge to keep the sliced open plastic backing from opening enough to let the carbon fall out when I don't want it to (such as when it's in the filter with water flowing through it).
I also rinse out the filter floss that makes up the other side of the cartridge.
And there you go, new carbon in the filter.
I mainly just use carbon for removing meds and tannins from the tank water now but I tend to change it before I put it in to remove meds/tannins.
If I were to use carbon all the time (instead of just when I need to remove meds/tannins) I would replace the carbon when the tank water starts to look like the carbon is not doing it's job (such as if the tank water looks discolored) or if the tank water starts to smell bad.
EDIT: A media bag should also work.
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