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.