Depending on brand, type etc sponges can last a long time, months even years. Just clean them well every 2-4 weeks (or whenever they get clogged). Carbon should be changed every month as it becomes exhausted and can leach things back into the aquarium. Ceramic rings etc can also be used for a long time.
I have to slighly disagree with TropicalNorth. I have an Aquaclear70 and haven't changed anything. I don't use carbon as its effects are negligable unless you are trying to remove medicationas from the tank or have very smelly water. By changing cartridges, you are destroying the beneficial bacteria. If you notice that your filter is becoming clogged you can lighly swish the filter material in the tank water but never actually take it out ans clean it. This will cause the tank to go back into a cycle. Though you should clean teh intake tubes and impeller on a regular basis, once a month or so is what i do.
I swish mine around in old tank water every water change, and only replace them if water just won't go through or they are falling apart. Usually its the latter.
pretty much with what clawclan says.
I rarely use carbon(exception is to remove meds,)
when cleaning the filter..i try to rinse the sponges( I have several AC filters)
about once every 2-3 weeks..but I use an extra sponge in lieu of carbon.
I Only rinse one sponge(the bottom sponge on an AC) and rotate the top sponge to the bottom. I do not rinse both sponges)
if you clean the entire filter including the sponges you may deplete the bacteria and you may see a mini cycle.
on my emperor filters I rinse the filter materials but rarely do anything to the bio-wheels)
i dont use carbon. the rest i clean here and there as needed. i havent had to get a new sponge or bio media in a very long time. as long as its not falling apart no need to change it.
Or you can just cut the carbon out and save it for a rainy day like when you have to remove meds or when something icky(chemicals) gets in the tank. That is what I am doing now. This method will also save you lots of money in the long run, not having to replace filter pads every month and sending your tank into a mini-cycle.
Odd...I use carbon all the time...it's effects are not negligible...it is not just for smelly water or to remove meds. We obviously need to have an online water chemistry course.