Crying unconsolably. A common definition of colic is an infant who cries for more than 3 hours a day, 3 days of the week, for more than 3 weeks. Although no one knows for sure what causes it, colic is fairly common, starting a few weeks after birth. Luckily, this condition goes away by about the third month of life. The main thing is to make sure your baby's crying is not caused by a serious condition.