Breastfeeding for over a year (cumulative time) lowers your risk of high blood pressure, diabetes and heart disease in your menopausal years. Relative risk reduction ranges from 10-20% depending on what outcome you're looking at. This is data from a very large population study called the Women's Health Initiative. Shorter amounts of breastfeeding are also protective, but the significant benefits are seen in women who BF for over a year of their lives. So supporting women in their attempts at extended breastfeeding is good for the national health and will reduce Medicare expenditures in later years!
Here's a summary in the NYT:
http://www.nytimes.com/2009/04/22/he.../22breast.html