The best time to start building a supply of frozen breast milk is a few weeks before returning to work. The ideal time to work in a pumping is generally in the morning since that is when milk supply tends to be highest. Try to time this pumping for at least an hour after baby has last fed…and hopefully about an hour before you expect him to nurse again. This will allow your body to produce enough milk for you to pump and also to replenish your supply before baby goes back to the breast. (Remember, empty breasts make milk faster than full breasts.) When they are pumping between feedings like this, most moms find that they're able to get somewhere between a half-ounce and two ounces. This is less than you'll be getting when you're at work, pumping in place of feeding your baby.
And you don't have to worry that your baby will wake up hungry right after you finish pumping. Feed her alternating between your breasts several times — as there will still be some milk there for her. She may want to feed again sooner than she would have otherwise but she won’t go hungry.
By adding in this extra pumping every day, or even every other day, you should build a good stockpile of milk in your freezer by the time you return to work. The more milk you have stored away, the more peace of mind you'll have during your transition back to work.