There is probably not a real good answer to how often. Cats are very fastidious, and if they detect an odor, they may avoid it. But my cats seem to concentrate on one or two litter boxes and leave the others alone, so I have to wonder what their 'standards' are. With multiple cats, that can really pile up in just one or two litter boxes.
Usually, even clumping litter doesn't come out totally, as little pieces break off. They are usually too fine to be gathered by the wire scoop. When those start to add up, I change the litter by allowing the level to go down, then discard and clean the litter box, and add new litter.
I have also dumped the litter out, cleaned the box, and returned the litter and added more fresh litter. They seem to accept that very well. I do that when they seem to soil the litter box about the litter level. And no, cats do not always sit perfectly in the center of the litter box.
When you do decide to clean the litter box, avoid strong scented cleaners as they may leave a scent that your cats may not like. If you use bleach, make sure the bleach smell is gone before adding litter.