Although crying is perfectly normal for a baby, it can be distressing for you and her. Colic, or extreme crying for no apparent reason, typically occurs between three weeks and three months of age, lasts more than three hours a day, at least three days a week and at least three consecutive weeks. Doctors don't know what causes it, but it usually goes away by itself by the time she reaches three months. In the meantime, try calming her with soft talking or singing or a gentle ride or walk.

Source:
"Colic," http://www.mayoclinic.com/health/colic/DS00058