Here’s what recent history tells us:
If you bought a market portfolio of stocks and reinvested the dividends, your odds of losing money in any one particular year were 32 percent. Your odds of losing money over any 5-year period dropped to 13 percent, and for any 10-year period the odds of losing money fell to only 2 percent. There has never been a 15-year period when stocks lost money.
That’s all you need to know.
-The Bogleheads’ Guide to Investing