The first time I returned to work while BFing for my daughter, I returned to work when she was 2mnths old. I tried pumping throughout the day to get enough milk and it wasn't really much. I ended up only having enough reserves for one day.
Second time around, I started pumping EVERY morning, on one breast and had the baby nurse on the other. Not at the same time though. I noticed that I was REALLY full when I woke up early, so I'd nurse my son on one breast early and he was satisfied and then Pumped the other breast while he was in a swing or bouncy.
Your supply will adjust to these needs if you don't see much at first. It's hard but keep it up. Some other things to encourage milk supply is eating oatmeal every day and you can try Mother's Milk Tea which they sell at nutritionsmart.
I did this every day and ended up pumping that one breast, but never returned to work but ended up with 100+oz after a few months and was shocked. I had to stop that every morning pumping session. It will build up. At first I only pumped 2oz from that breast in the morning and then it boosted up to 4 oz.
It has to be in the morning, because that's when you'll have the most milk. Also most babies prefer one breast over the other, so make sure to breastfeed your baby on the breast he prefers and pump the other, so he doesn't get fussy. Also when you nurse on one breast only, the baby will get all the hind fat, and will get fuller. :)
It also helps to pump consistently at the same time. And I would only recommend pumping that one time in the morning, vs. after every feeding. When I did both, i Noticed I did a lot better with qty just pumping once in the morning, on that one breast and feeding the baby normally the rest of the day.
-N.