Until it falls apart... or stops performing how you expect it to
Exactly !!! Don't believe the manufacturers "recommendations" about needing to change it every so many months. Save your hard earned cash as it's just a ploy to spend more money.
Replaced mine once in 5 years on a xp3. They tend to shrink before breaking down from what I've seen, which depending on the the filter can be a problem.
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Filter pads eventually lose their integrity but foam last for a reeeeeeeally long time. I've never changed the media in my canisters, but I am going to be swapping out the pads in my sunsuns for foam. Pads are about 3 years old and have started to not fill the tray as they used to.