It has never occured to me to change them every 6 or 12 months, they get changed when they look like they need it - but I have way more trays than I do cats, there are only my cats that religiously have the same trays, and admittedly the Lidl ones could do wtih being replaced, teh pink litter did stain them slightly, but if I give Tiger a normal tray she gets litter everywhere, so she shall have to wait!!