How long should I pump?

Kara • 🤍✝️🤍

My baby is 4 weeks old. I pump every 3 hours and get about 5.5-6.5 oz. I used to pump for 30 mins but I noticed that by 20 mins no more milk comes out so I stop at 20 mins now. Should I keep pumping for 30 mins? Will it help to eventually increase my supply?