Experts advise adding litter to the box after each time you scoop to replenish what you've removed. "Don't raise the overall depth of the litter as cats prefer their litter to be a bit shallower than you might think," LaFontaine said. Dr. Barrack noted that the ideal litter height is two to four inches, or more for cats who like to scratch to the bottom. She also stressed the importance of changing the litter out in the box. "Generally, I suggest replacing litter about twice a week, but if you clean the box daily and use clumping litter, you may be able to wait until once a week or every other week to replace it," she said. This will help prevent odour build-up. "A very thick layer of clumping cat litter will help prevent urine from reaching the bottom of the box where the odour can be hard to remove," Dr. Coates said.