Nancy Mohrbacher has written about the "magic number" and milk supply maintenance:
http://www.nancymohrbacher.com/blog/2010/8/13/the-magic-number-and-long-term-milk-production-part-1.html

In her book, Exclusively Pumping Breast Milk, Stephanie Casemore suggests that women typically pump for 120 minutes per day for as long as they pump (meaning even when they drop a pumping session, they pump longer at other sessions and still maintain that 120 minutes of breast stimulation). The book is really worth reading - it helped me to understand better how to support moms who are pumping for all, or even some, of their babies' feeidngs.

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