I think how often to change the litter depends on how many cats share the box, how much they go, and the type of litter that you use.
I have one very old cat that sleeps most of the day. The litter box is used maybe once or twice a day an not that much. I clean the litter box about twice a week on average and the litter isn't that dirty.
When this cat was younger I also had another cat in the house using the same box. I had to clean it at least once every day and sometimes more.