How can you tell if you need to replace your cartridge filter or if you can just clean it?

You can continue to clean the cartridge until it begins to have worn places, tiny holes or other obvious damage. If it gets to the point that the pores of the fabric are clogged it needs to be replaced. You should be able to monitor the pressure guage. There should be a noticeable difference before/after cleaning, if not it's too clogged.