Lockman is being forgiving saying the carbon loses effectiveness in a few weeks - on average it really is used up by about 3 days. Most freshwater hobbyists just keep it around on their supply shelf to remove medications. Now, leaving it in is not necessarily all that bad in the short run as the bacteria will attach to it just like anything else. It is just not as optimal as the sponges and ceramics whose primary purpose is being "biomedia" (carbon being "chemical" media.. and floss pads being "fine mechanical" media) Those three categories (biomedia, chemical media and mechanical media) represent the three fundamental functions of filters. For biomedia, ceramics have the advantage of lasting virtually forever, sponges are just as good at supporting biofilms but do tear eventually and must be replaced (a third at a time to keep the colonies going.) Sponges have the advantage of being lighter weight and maybe a little easier to work with.
Nearly all the media you use will play some role in "mechanical" filtration (the trapping of particles) with pore sizes determining which size particles will be trapped where in the filter. Ceramic rings play a good role at the beginning of the water path by slowing down and randomizing the little flows of water. Water that moves more slowly over a bigger volume of media will be filtered better because each media function has a longer dwell time to carry out whatever it's supposed to do (drop its particles in the case of mechanical, get stuck based on chemical charge in the case of chemical media, get processed by bacteria in the case of biomedia.)
If you keep your carbon in to use as biomedia you will just want to eventually go through a process of gradually replacing it with ceramic or sponge (so that you don't lose too many bacteria as you swap out) since in the longer run the carbon will gradually crumble (this is a major reason it is not as good a biomedia as ceramic or sponge) and be carried away in weekly water changes as tiny dustlike particles.
Hope this helps a little.
~~waterdrop~~
