Crying is normal for babies, as it is the only way they have to express their needs. Yet roughly 15 to 25 percent of infants cry excessively with no apparent cause. Colic is generally diagnosed by the “rule of threes”: crying bouts start at around 3 weeks of age and last for 3 or more hours per day, 3 or more days per week, for more than 3 weeks in a row. Though it is little consolation to tired and frustrated parents, colic usually subsides by 3 or 4 months of age. Here is what you need to know about this common condition.