I have a decent supply but since I went back to work I have had a hard time pumping enough in the time frame. I need 4oz per pump session and I'm only getting 3 oz in 15-20 min. The problem is after 20 min I am still producing but I don't have time to pump any longer. How do I pump quicker?

Side note my baby is allergic to milk and I just found out the formula he would need is about $80 a week! So now I feel stressed and worried that I have to pump for a year! My baby is 4 months.