This depends on if you want to use carbon or not-most of the carbon/activated charcoal is only active and useful for about 2-4 weeks
If you want to use carbon you have to change it at least monthly-
Depending on the type of filter media-if it is the type that is a combo sponge with carbon in it or if the sponge and carbon are separate
Combo-you can cut the old sponge away and attach it to the new sponge for a week or so to seed the new sponge
Separate-just replace the carbon
Rinse the sponge in old tank water with water changes 1-2 times a month and when the flow slows to get the big pieces of gunk off to maintain good water flow-you want the sponge to look dirty-this is the good stuff that is needed.
I don't use carbon/activated charcoal-not needed but nothing wrong with using it either-it is a personal choice more than a need IMO/E- what carbon does is help keep the water clear, removes tannins, medication, odors...but so will a water change-use of carbon does not change the fact/need of regular water changes
I make my own filter media with poly-fill I get in the craft department for less than 5 buck a bag and it will last a long time-I use rubber band with some of the filters and others I just cram it in my filter box-