I have one box for each indoor cat. I use a clumping litter, scoop the boxes daily, and wash them out every 2-4 weeks, depending on the condition of the litter & box. I dump the litter out completely, rinse the box thoroughly, and fill it with hot water & a little bit of bleach & let it soak for a little while (keep the cats well away from this - some are attracted to bleach). Then I rinse the box thoroughly, let it dry in the sun, and return it to use.
How often you wash the box may depend on how many cats you have, and also what type of litter you use, and if your cat gets the sides dirty, or digs hard (this tends to break up clumps & makes the litter dirtier faster).