It is recommended to change the liner as often as needed. If the cat litter box is in a confined space or if it is used in a place with high foot traffic, frequent changes can be a good idea.
It's important to clean your cat's litter box weekly, but if you have more than one cat you may need to clean it more often than that. Be sure to scoop out all of the clumps and solid waste at least once every day and change the litter every few days or so, depending on how many cats you have and how often they use the box. The frequency of changing your pet's litter box will depend on how often he uses it and where it is located.
If you're using a disposable liner, change it at least once every two weeks. For permanent liners, use them until they start smelling bad, then replace them with fresh liners. You don't need to wash the whole box if you do this; simply wipe down the outside with soap and water. To make sure there aren't any odors lingering after cleaning, give the box a good airing by opening all four doors and letting air circulate through the box.