Breastfeeding Moms: How long can you go without feeding or pumping before milk supply is effected?

Nancy
My girl is sleeping long stretches every night (6-8hours). I don't want to keep pumping in the middle of the night but I don't want my milk supply to suffer because of it. What are the opinions on how long can you go without feeding or pumping before milk supply is effected? And when you pump, how much/long do you have to pump to prevent losing supply?