I heard is 4 to 7 days.
My daughter is 2mths old too, on mixed BM and MF facing the same problem. She gets very cranky after many days of not able to poop. Now abit better, I have been doing the massage on the tummy (gentle circular clockwise direction and the bicycle exercise), diluting the MF (Less ¼ scoop) and offer as much BM as possible because my BM supply is very low.
Dr don't encourage to dilute the MF too much and take note of any tense uncomfortable tummy. Can try changing the MF but I didn't because I am worried about rejection and other problems.
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