You've gotten a lot of great advice. In case nobody mentioned it, you want your filter cartridge to get dark and nasty looking. Should start smelling a little swampy too. Shake it off in the bucket of aquarium water like they said but keep putting it back in.
I'm finding that in my small filter, while I can't fit 2 cartridges I can fit am extra square of filter media. I buy pads meant for canister filters and cut them down, rubber band them and whatnot. Using a water polishing pad behind my filter cartridge makes my water super clear even though the charcoal is likely work out so to speak. And charcoal that's all work out is super populated by bacteria, so you want to keep it in there!
I've read that instead if cartridges people also just put in wadded up filter floss (or poly fill that isn't treated for flame resistNce). The more I experiment with my filter the more I realize I can't really mess anything up as long as I don't throw away my bacteria colonies!