Filter cartridges should be changed when they are, literally, falling apart. When the flossy/fuzzy stuff starts to break down to the point that the carbon is falling out...then they need to be replaced. The problem with those cartridges is that when you replace them, it either gives a huge bump to the cycle, or can even crash it. So, when you do replace, leave the old one in the filter for a couple of weeks with the new one so that they can colonize one another with BB's.
The alternative is to just ditch the cartridges. This is what a lot of people do, you don't need the carbon, and it can absorb nutrients and trace minerals that your fish *need*...so they can actually be harmful in some small ways. If you decide to get rid of the cartridges all together, you'll just need to stuff the filter full of aquarium sponge or filter floss. Just cut to fit, and use in place of the cartridge. :) It actually works *better*, because it's much more surface area for the BB's to live in, and the sponges last just about forever, so much cheaper. :)
Your filter shouldn't need to be cleaned on the inside. Eventually, if it starts to get gunky, just take the filter sponge and rub it down a little with old tank water. Then just rinse the sponge in the tank water as well and you're good to go. Never use any chemicals or any straight tap-water on any part of your filter, filter cartridge, or on sponges because that will kill your BB's. :)
Does this make sense? If not, PM me just in case I don't come back to this thread. :)