Pumping

I'm exclusively pumping, how long do my pumping sessions need to last and how often should I be pumping? Hoiw many oz should each breast be producing? I was making about 3 or 4 on each breast and now it seems like 2 to 3. I don't know what else to do to get my supply up. I'm pumping every 2 to 3 hours but not pumping throughout the night, could that be my problem? Thanks!!