Carbon filters should be changed once a month-
Sponge filters replaced when it is falling apart-
To clean out the sponge media in both the carbon and sponge filter, remove it during a water change and swish it in old tank water (water you just removed from the tank during cleaning) to remove the sponge of debris- and then place back into the filter.
When having to replace the sponge part of either filter type, add in the replacement a week or two prior to changing it out to give it time to build up the proper bacteria. Normally you only have to replace the sponge once, maybe twice a year, and sometimes not even then but longer.
Gravel will hold a lot of the beneficial bacteria, so your tank won't have to cycle itself when you replace a filter media such as the carbon- and normally there is a sponge part in the carbon filters as well.