I went back to work 6 weeks post partum and I had to pump every 2 hours, on both breasts, for at least 15 minutes (or until the bottles were full). Each time I pumped, I was able to get at least 4 ounces per breast. It got really tiring and started to interfere with my production at work. My employer was open to my pumping at work, but thought I would pump once or twice a day. I was just so scared that i would start to make less milk if I pumped less. Needless to say, I did have to decrease the amount I pumped during the day but my milk supply only decreased a small amount.
I think I could have pumped less if I had invested in a more powerful breast pump. My friend had a really good one and she only pumped every 4 hours and was able to get 3 times as much as I got. The only thing, your breasts will supply milk based on demand. So if you only pump every 4 hours, milk will only come down to nurse baby that often. At least that's what they say.
I pump twice a day for about 10 to 20 mins. Early morning and right before I go to bed. In the morning it is about 30 minutes after she had her early morning feeding. The night is around 10pm. She is down for the night btwn 8 and 9pm. That is how I pump.