If you are buying an expensive pillow, they probably come with advice on how long it can last, sometimes up to 5 to 10 years. However the cheaper ones, like normal hotel pillows, should be changed once they are losing their bounciness, which is usually within 1 year. When a pillow loses its bounciness, it is unable to support the neck any more. One way you can try to extend the lifespan slightly, is to turn the pillow from time to time so that you are using different areas of the pillow.