The carbon has a very limited effective lifespan. After a week it does very little but still works to some degree for another couple of weeks. I would personally change the filter every 3-4 weeks but rinse it out with each water change. If I understand you correctly you are using the type of filter pad that has the carbon and filter pad as a single disposable unit. If you have one with a sponge and carbon separate then just replace the carbon and rinse out the sponge as the sponge will last a very long time.
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