There are differing opinions regarding the answer to this question. However, the American Cancer Society, the American College of Radiology, the Susan G. Komen for the Cure Foundation, the American College of Obstetrics and Gynecology, the Society of Breast Imaging and the National Cancer Institute all recommend at least biennial (every two years) mammography beginning at age 40, with most recommending annual mammography.
Read Dr. Jerome Schroeder's response to the USPSTF Guidelines here.