Typically, you can use 1 cup per 100 gallons of water for normal use, and change it out once a month. If you are trying to remove a chemical that was added to the water (like interceptor to treat red bugs) then I would still use the one cup per 100 gallons, but change it a few times soon after treatment.
If you get 25# of carbon, be sure to store it correctly in an air tight container as carbon will absorb impurities from the air, and possibly add them to your tank later.