"The average annual total return and compound annual growth rate of the S&P 500 index, including dividends, since inception in 1926 has been approximately 9.8%, or 6% after inflation" [0]. If you have a 40 year career, money you put in at the start would approximately be worth 10 times what you put in adjusting for inflation, and even money you put in midway would approximately be worth triple.

[0]: https://en.wikipedia.org/wiki/S%26P_500_Index