If you are finished with the treatment of the condition for which you were using the medication, then yes you need to use activated carbon in the filter to remove the residual amounts of medication left after the water change. It should take between 4 - 8 hours to remove the rest of the medication depending on the type of filter and size of your tank.
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I'd keep the activated carbon (in a tank that was medicated) for a few days. Activated carbon is good only for a few days, maybe a week. So just keep it there until it expires (say, up to 2 weeks). This way you'll make sure that as much as possible of the leftover traces of the medication is removed from the water.