Pumping moms...

How often and how much milk are you getting per pump session? 
My son is 4 weeks old, I pump every 3 hours, except for night when I go for 6 hours between. I get about 5 oz total per pumping session. 
   It's so exhausting pumping so frequently. When can I decrease the amount of sessions??? I'm going crazy, plus, my son nurses too.