The window of opportunity

If you're going back to work and need your baby to be able to take a 
bottle, then it's important to introduce them to a bottle before 6 
weeks.  There's a window of opportunity between 3 and 6  weeks when 
most babies are flexible and can continue to breast feed and take 1 to 
2 bottles of either pumped milk or formula each day. If the bottle is 
introduced too late, say after 8 weeks, then most will refuse it and 
may even go on a hunger strike.

It's also important to offer a bottle at least once each day, so that 
the baby will develop a routine. Sometimes, if there is more than 3 
days between bottle offerings, a baby will suddenly refuse to take 
it.  Here's a few more helpful hints:  Sometimes a baby will not take 
a bottle from a  breastfeeding mom, preferring to breastfeed, so other 
family members or partners need to be available.  Sometimes, babies 
will not take a bottle if mom is in the same room, apparently they can 
smell that mom is there and may not be interested in a bottle.

Every baby has their own temperament.  Some will move easily between 
bottle and breast and others are more stubborn.  In any case, keep 
trying, so that when you do go back to work, you'll feel more 
comfortable knowing that your baby can eat.