About 2 weeks. Since the species isn't mentioned, I will assume human. Definitions Colostrum: This is the fluid expressed by the breasts just prior to and for a few days after giving birth. It contains a much higher concentration of nutrients in an easier to digest form than regular breast milk. This type of breast milk will be produced for 2 to 4 days after giving birth. Transitional Milk: This is a thick creamy milk that contains more fat, lactose, vitamins and calories than colostrum. It starts within 4 days of giving birth and lasts approximately 2 weeks. Mature Milk: This is a thinner milk that's about 90% water and 10% nutrients. It starts about 2 weeks after giving birth. So the answer is about 2 weeks.