When to start adding in pumps if you are going back to work is really up to the mom and what her milk/feeding plan is ideally.  I say "ideally" because we all know baby runs the show! 

If you want to maintain breastfeeding and pumping once heading back to work practice  pumping at home the same pumping schedule you plan to have at work for the week or two before you go back, so your body and baby are adjusted.  See if you can have your lunch and break time at work spaced similar to when your baby normally eats. Continue to nurse in the evenings and on weekends.  This method has you pumping for the next day's supply at day care and requires the least amount of storing and pumping.

If you plan to pump exclusively or switch the majority of your sessions to pumping once you start working, start pumping 4-6 weeks before heading back to work.  Once or twice a day add 5 minutes of pumping in after your baby is done nursing. You can start with any session, but I highly recommend starting with yomorning session when your hormone levels are higher.  These extra pumps will signal to your body to make more milk and you'll start to collect any milk in your breast that your baby didn't finish. Continue to pump after every few sessions,  then 2-3 weeks prior to heading back to work (or when you have a little stash of milk built up) you can start swapping feedings for pumping sessions and transition in bottle feeding.

Regardless of the method, it's also a good idea to start bottle feeding at home a week before during the day when your baby will be getting a bottle.  This often helps ease the transition into daycare, so you baby doesn't have to adjust to body feeding and being away from mom at the same time.