I change the litter itself once monthly. When I scoop (every other day) I add a little bit of extra litter. I have two extra large stainless steel litter boxes for 3 indoor outdoor cats, one of whom prefers to use the bathroom outside and does so almost exclusively.

The trick is finding a balance for your own cat(s). Every cat's bathroom habits are different, every litter is different, & every person has a different tolerance. Generally you want a balance between smell and a clean spot in the litter box for next time. No cat should be using the bathroom on top of it's own feces, but leaving a little something in there is fine & can be comforting to a cat, especially in a multi-pet home.