Depends on who you ask and/or how you do it. If you actually change it, you should change it once every other week at the longest. Every week would be better. The reason is so that the bio filter establishes in other places of the tank, so you aren't removing it when you change the floss. If you don't want to change it that often, your best bet is to never change it, and just rinse it out once in a while with old tank water after a water change.
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