During periods it is really important to be sure about your personal hygiene. To prevent certain diseases like UTI's or yeast infection cleaning yourself is needed, in normal days as well. Hence during those days, it is very important that you change your napkin often frequently to avoid any diseases.
There are some questions which are not answered. So let's talk about them:

HOW OFTEN SHOULD YOU CHANGE A PAD?
Once in four hours, you should change your sanitary napkin, according to health experts. If you are using tampons apart from napkins then they must be changed once in two hours.
*The above time depends on your personal requirements. Some women might have a lighter flow or few have heavy bleeding.

WHAT ELSE SHOULD BE KEPT IN MIND? 
Everone knows about this phenomenon that once the blood has left your body it starts getting contaminated easily. Not in your heavy days but in light also. Once the pad is damp, the organisms tend to get multiply in moist and warm conditions leading to UTI's or vaginal infection etc. If you are using a pad of a branded company then you can extend the time to an hour but not more than that.

HOW MANY PADS CAN YOU USE AT A TIME?
Avoid using two sanitary napkins at a time. On their heavy days, many women's use napkins with tampons or a pad and cloth etc, which is not a healthy option. Using two types can absorb more amount of blood from your body and you are unlikely to change them at regular intervals. This process can lead you to infections or vaginal rashes.

THE IDEAL WAY:
As told above change your napkins at regular intervals. After using the washroom or after changing your pad wash your hand properly to avoid bacterial infections and if you find a pad rash or burning sensation when you pass urine than seek medical help.