A 58-year-old woman sees her naturopathic health care provider for a routine health examination. There are no remarkable findings on physical examination. A screening mammogram shows a 0.5-cm irregular area of increased density with scattered microcalcifications in the upper outer quadrant of the left breast. Excisional biopsy shows atypical lobular hyperplasia. She has been on postmenopausal estrogen-progesterone therapy for the past 10 years. She has smoked 1 pack of cigarettes per day for the past 35 years. Which of the following is the most significant risk factor for the development of lobular carcinoma in patients with such lesions?