My first one was probably dairy-sensitive, although at the time I didn't recognize the signs. It lasted until he was around four months old, I think. My second was rabidly dairy-sensitive, and when she was six weeks old we went to the pediatrician, who got me on a no-dairy diet right away. I found I needed to stay on that until my baby was about six months old, which I think was kind of an extreme case. While I was still on dairy she was not pooping at all most days (and when she did it was green and mucousy); she was projectile-vomiting several times a day; and she would scream in pain almost constantly. Her dairy intolerance was very severe, which, I think, is why I had to avoid dairy for so long.
Mama to Thales, 4 1/2, and Lydia, 18 months