Exclusively pumping

So my son is almost a month and I've been exclusively pumping and bottle feeding. Breastfeeding took too much out of me so I only did it for about a week. Just wondering if I'm pumping every 2 hours, how long should I be pumping at a time? I usually do every 3 hours for 20mins and get about 8oz total each time. 
Also any tips advice on exclusively pumping? Thanks! 😊