More recent data published regarding long term usage:

"Long term continuous administration of natural and synthetic estrogens in certain animal species increases this frequency of carcinomas of the breast, cervix, vagina, and liver. There is now evidence that estrogens increase the risk of carcinoma of the endometrium in humans (See BOXED WARNING).

At the present time there is no satisfactory evidence that estrogens given to postmenopausal women increase the risk of cancer of the breast, 18 although a recent long-term follow-up of a single physician’s practice has raised this possibility.18 Because of the animal data, there is a need for caution in prescribing estrogens for women with a strong family history of breast cancer or who have breast nodules, fibrocystic disease, or abnormal mammograms."

Source: DailyMed