Your breast will never be completely empty, because milk is always being produced
Here's a great explanation from kellymom.com:
Do breasts need time to refill?
Many people mistakenly think of a mother's milk supply as being like "flesh-covered bottles" that are completely emptied and then need time to refill before baby nurses again. This is simply not how we understand milk production to function.
First of all, milk is being produced at all times, so the breast is never empty. Research has shown that babies do not take all the milk available from the breast - the amount that baby drinks depends upon his appetite. The amount of milk removed from the breast varies from feed to feed, but averages around 75-80% of the available milk.
Trying to completely empty a breast is like trying to empty a river -- it's impossible, since more milk will keep flowing in while milk is being removed.
Research also tells us that the emptier the breast, the faster the breast makes milk. So when baby removes a large percentage of milk from the breast, milk production will speed up in response.
Rather than thinking of nursing or pumping as "pouring milk out of a container" think of it as flipping on the "high speed production" switch!
Yet another analogy: Imagine you are using a straw to drink from a glass of water. As you drink, a friend is very slowly pouring water into your glass. The emptier the glass, the faster your friend pours the water. Would you be able to drink all the water in your glass?
(
http://www.kellymom.com/bf/supply/milkproduction-faq.html)
And if you're interested in the science behind milk production:
http://www.kellymom.com/bf/supply/milkproduction.html
