Originally Answered: how much Kitty litter in the tray?
Great question. It's important that cats have access to a clean litter tray at all times, so the cleaner you can keep their trays the better! The usual recommendation is to scoop at least twice daily (remove contaminated litter) but this can be more difficult with non-clumping litter (e.g recycled paper litter) which may need to be completely replaced every day. The litter tray should then be emptied, cleaned with a mild non-toxic detergent and dried approximately once a week.
Cats also have individual preferences for the depth of litter that they prefer in their tray but it should be enough for them to be able to dig and cover their business! Most tend to prefer a depth of 1-1.5 inches but some will prefer deeper or more shallow litter - so it's good to get to know your cat's preferences. It's also important to have one clean litter tray available per cat in the household, plus a spare (e.g 2 cats require 3 trays) so there's always access to a clean tray.
Hope this has been helpful!
Dr. Ellen Marcinkiewicz BVSc. MRCVS