I started back to work when ds was 4 months. He's now almost 10 months & I've been pumping... gosh... for almost 6 months now. He hasn't had formula at all this whole time.
In the beginning, his & my schedules worked out so that I could use my lunch break to bf him & I'd just pump in the morning & afternoon (~10 & ~4). Recently his schedule changed so now I'm pumping at work at 10, 1, & 4.
One thing... when my ds started dropping feeds (solids, sleeping thru, etc), I had to continue pumping as though he were feeding at those times. If I dropped below ~7 "demands" on my breasts (nursing or pumping), my supply seemed to decrease. So now I pump during the day & a time or 2 after ds goes to sleep. I actually nurse in the morning and at bedtime. HTH.