What you can do is just swish the cartridge in old tank water you just took out from a water change. Swish and rub around in the water and put back in. This gives your cartridge more life. Companies say to replace once a month but honestly that's really just a ploy to get more money. My cartridges have lasted near 6 months now and I'm only debating changing them out for better filter media.
However you must remember that most of your colony of Beneficial Bacteria (BB) is living in your filter cartridge/media. Soooo in order to get your BB over to your new media, it's advised to try to stuff both the new and old cartridges in the filter. If they don't fit, then you can basically tear the old cartridge apart (careful there is carbon inside most and it's just a pain to pick up if it goes everywhere) and you can basically just put it on the new media but make sure to put it on the backside so that the water flows from old media to new media and then to carbon floss if you have it. This way your BB can safely move to your new media and you don't have to worry about cycling your tank again or anything.
And then after a week or two you can take out the old scraps and the new cartridge is good to go for a while! But just swish it out at each tank cleaning and it will stay fairly clean