As often as it takes to avoid overflow! How often you need to change your pad depends on how heavy your flow is and what type of pad you are using. Pads come in all different absorbencies, so match up the pad absorbency with your flow. On your heavier days, you may need a more absorbent pad and on our lighter flow days a less absorbent one will do. No matter how heavy your flow is though, you should change your pad at least every 3 or 4 hours so you don’t end up with an odor from bacteria that can grow in the blood. Yuck!
As often as you need to. The truth is, the length of time you can wear a pad really depends on how heavy your flow is. I usually have one ridiculously heavy day each period. On that day, I may be running to the bathroom almost every hour to change my pad. On lighter days, however, that would be overkill. Even if you're not soaking the pads, aim to change it every 4-6 hours. A new pad will help you feel cleaner. Plus, while your period blood itself doesn't have any odor, changing your pad frequently will help prevent odor in general.