http://www.ssa.gov/retire2/index.htm


Quoting here
Your Social Security benefits are the foundation on which you can build a
secure retirement. Most financial advisors say you'll need about 70 percent
of your pre-retirement earnings to comfortably maintain your pre-retirement
standard of living. Under current law, if you have average earnings, your
Social Security retirement benefits will replace only about 40 percent, so
you'll need to supplement your benefits with a pension, savings or
investments.