According to Vanguard, the average annual return on stocks from 1926 to 2013 was 10.2%
https://personal.vanguard.com/us/insights/saving-investing/model-portfolio-allocations
That is a nominal return. The real return (i.e. inflation adjusted) is about 3 points lower. A nice data source for real and nominal S&P returns with dividends is here: S&P 500 index - Bogleheads