Generally you want to use that cartridge until it's literally falling apart. So every week or every two weeks, take the cartridge out to give it a swish and rub in some conditioned water. That will help with longevity.
When you get a new one though, I know these filters and the cartridges both won't fit in there well, so if you can gut the old one and stick the cloth material on top of the new cartridge, or rubber band it there to stay, that will help bring over some Beneficial Bacteria to the new cartridge.
But I've had cartridges that I set up in December that I'm still using and they haven't needed replacing yet. So just keep an eye on it, wash it out every now and then, don't let it get dry and you should be set