I've heard that rule too, but we only have 2 boxes for 3 kittys and that seems to be plenty. They are the huge boxes, since both Lucky & Fluffy are both around 20 lbs each. I clean it out nightly and monthly we actually clean & disinfect the boxes (unless someone decides to dirty the box!) Plus I'm usually in there after "someone" uses the box to cover and spray.....P-U!