Depends on what shape the filter is in really.
You can certainly rinse the filter out in the tank water you remove during water changes and just reuse the filter. Sponge filters will last about forever. The ones made of floss of some type don't last anywhere near as long.
I have the floss ones right now (I'll be replacing them with sponges as soon as I find a type large enough for a Emperor 400) and I rinse them. There does reach a point however, when the filter isn't very useful any more. I just change mine completely when the filter media starts getting ratty; that can be anywhere from 2 weeks to 2 months in my case (mine get pretty groody; thanks to the piggies in my tank). However, I do have 2 mature bio-wheels, so I could replace both filters each water change and not spike any ammonia.
In your case it shouldn't be too much of a problem replacing them (whenever you decide to do that) as long as the tank is fully cycled. Keep in mind the bacteria colonise all surfaces: the filter media, the filter itself, the substrate, the tank walls, the plant leaves, etc. I don't suggest doing a gravel vac AND a filter replacement at the same time tho. Btw, you might want to squeeze some of the ook from the old filter onto the new one to "prep" it.
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