Women who spend 1-3 hours per week exercising appear to reduce their risk by about 30% when compared to inactive women. With a steady level of 4 or more hours of physical acitivity per week the relative risk can be cut by at least 50%!

Bernstein L; Henderson BE; Hanisch R; Sullivan-Halley J; Ross RK. Physical exercise and reduced risk of breast cancer in young women [seecomments].Journal of the National Cancer Institute, 1994 Sep 21, 86(18):1403-8.(UI: 94351760)

Abstract: BACKGROUND: Epidemiologic evidence strongly suggests that cumulative exposure to ovarian hormones is a determinant of breast cancer risk. Because physical activity can modify menstrual cycle patterns and alter the production of ovarian hormones, it may reduce breast cancer risk; yet few epidemiologic studies have assessed this relationship.

CONCLUSIONS: Most previously identified risk factors for breast cancer are reproductive and menstrual events that cannot be readily altered. The protective effect of exercise on breast cancer risk in the women whom we studied suggests that physical activity offers one modifiable lifestyle characteristic that may substantially reduce a woman's lifetime risk of breast cancer. IMPLICATIONS: Whether the protective effects of exercise on breast cancer risk are due to alterations in ovarian function and whether they extend into women's menopausal years need to be established. Our results suggest that implementation of regular physical exercise programs as a critical component of a healthy lifestyle should be a high priority for adolescent and adult women.