The simple answer to how often you should change your panty liner is: when it's soiled or wet. But, let's face it, periods aren't always that simple. Depending on your flow, you might need to change your liner more or less often.
If you have a heavy flow, you might need to change your liner every couple of hours. On the other hand, if you have a light flow, you might be able to get away with changing it only once a day. Of course, it also depends on how often you go to the bathroom and whether or not you're wearing a pad or tampon as well.
The bottom line is that you should change your liner when it's wet or soiled. And, if you're unsure, it's always better to err on the side of caution and change it more often than you think you need to.