how often should I replace the metal filter on a Bialetti Moka Express 6-cup?

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You do not need to replace the filter that often, it is just metal after all. As long as you can clean it properly and it functions normally there is no need to replace it just because you're replacing the gasket, especially when there's no visible corrosion or damage.

There are some people who use their Moka Pot very often, don't clean it properly and almost neglect it. They can go through gaskets fairly quickly, my mother used to change it every few months, but she just replaced the filter just once, and that was because she lost the original one. For those people getting a few gaskets for a cheaper price per gasket makes sense. For others, getting just one is fine. I have even seen a pack of 6 gaskets and one filter not long ago.

Clean your Moka Pot after every use (or at least rinse it thoroughly and clean it properly every few days) and you filter should last you quite a long time.