Actually, most experts recommend that you have one litterbox for each cat in the house, plus one. That can start to be a lot of work if you have several cats, but it's still a good rule of thumb.
I usually scoop out the box several times a week, to keep the litter as fresh as possible. When I use clumping litter, I get all the clumps out and then add some fresh litter. That really helps stretch the litter, and still keeps the box fresh for the cats.
I use litterbox liners too, so when it's time to change the litter completely, I just bag up all the litter and tie up the liner. I dump any loose litter out, wipe out the box and put in a fresh liner.