Melly,
I would replace the carbon sponge with filter wool. This helps clean the water far better and can prevent algae outbreaks. Every week just take out the filter wool and clean it thoroughly under running tap water and reuse it in the tank. It can be replaced when it becomes uncleanable and hence should last atleast a month (if not more).
Same goes for other filters as well. I know that the manufacturers recommend that it be replaced every week/months etc. but in reality they can be reused for much longer durations.
Hope this helps.