With light period flow a period pad can sometimes last up to 8 hours without changing the period pad.
However it's not recommended to wear the same period pad for 8 hours due to the blood and bacteria building up on the period pad.
The period pad would get really icky and gross and could cause a rash or other infection due to the bacteria on the period pad that grows over those 8 hours.
I would recommend changing the period pad every 3 to 4 hours if possible to avoid infections and to keep yourself clean.
Some girls are embarrassed about having to carry extra period pads around and will wait until 8 hours later to change their period pad but that really is not very good hygiene.