The Flip Paperboard Liners may last up to one month. They are coated to prevent odor absorption and leaks. If your cat is not the liner type don't worry, Flip can also be used without a liner as it's seamless all the way around. 

Here is a video showing the paperboard liners being installed, @ 25 secs: https://www.youtube.com/watch?v=yZZrCWpok_w.