Most filters contain a mixture of the following.
1st usually filter wool. Quite fine. Filters out partical matter as soon as water enters filter. MAINTAINANCE: Squeeze out weekly in old tank water. Replace when falling apart or completely clogged.
2nd usually filter sponge. Sometimes just a fine sponge material sometimes a coarse and a fine sponge, depends on type of filter. MAINTAINANCE: Squeeze out monthly in old tank water or when completely clogged up. Dont over clean as some or your beneficial bacteria hang out here.
3rd usually carbon filter material. Not generally needed unless you want to remove medication or dicolouration from your water. Most people here replace this with more sponge or wool.
4th usually your media. Ceramic tubes or such like. Most of you beneficial bacteria hang out here. MAINTAINANCE: leave well alone. If they start to fall apart (takes years) then replace but only about a third at time and leave the new stuff in for a week or so to be colonised with bacteria b4 replacing the next third.
I clean my tank out weekly but then it's small and more prone to water quality issues. You may get away with a good clean fortnightly and water changes weekly.
My Maintainance Routine Weekly:
1. Switch off filter.
2. Remove large rocks / logs
3. Trim plants.
4. Siphon gravel + remove 20% of water with am Interpet gravel vac.
(Keep this water in bucket for cleaning filter wool and sponges)
5. Squeeze out filter wool in old tank water. Don't overclean it though as this is where the a lot of your bacteria hang out. Gunk is good.
6. Clean off worst of algae on glass and other bits, spray bar etc. with algae scaper.
7. Kill as many snails as possible.
8. Replace anything removed earlier.
9. Replace water with dechlorinated tap water. To warm this up either stick a spare heater in a bucket a few hours b4, microwave 3l sqaush bottles to warm your cold up or possibly add water boiled in kettle to cold to bring the temp up. (be aware of limescale thou.)
10. Clean evaporation cover below tank light.
11. Dose liquid fertiliser.
12. Clean outside of tank.
MONTHLY
1. Squeeze out filter sponges in old tank water. Again don't over do it.
2. More thorough clean.
EVERY 3 - 4 MONTHS
1. Take apart as much of filter as possible and clean thoroughly with old tank water.
Only replace filter sponges and wool if they are falling apart and even then only replace a third or so at a time and leave for a week or so before adding more.
Hope that helps. Lengthier than I was expecting.