Two to four weeks on the makers’ own guidance — CROPLAS states two to three — and carbon is spent long before it looks dirty, so the calendar is a better guide than the appearance.
Carbon adsorbs until it is loaded, then does nothing. There is no visual cue.
The interval
Two to four weeks is the range every maker in this family quotes. In a warm room with two cats, take the shorter end.
Why your nose is a poor guide
You adapt to a smell in your own house within minutes. A visitor notices what you cannot, and a filter that “still seems fine” is usually one you stopped smelling weeks ago.
Unscented, always
A perfumed insert covers odor and gives a sensitive cat a reason to avoid the box. Carbon adds nothing to the air, which is the point.
What a filter will not fix
A full drawer or a perished seal. Those are the first two sources of odor and a filter is the third — replacing it while ignoring the other two is how people conclude that filters do not work.
Cutting generic pads
A pad cut slightly oversize seats under tension and seals its slot. One that drops in loosely lets air pass round the edge rather than through it.
Last reviewed 2026-08-17