Jason Zweig, via WSJ:
Since 1929, the S&P 500 has suffered 14 bear markets, defined by S&P Dow Jones Indices as losses of at least 20%. The shortest and shallowest was the 20% drop that lasted less than three months in late 1990. The deepest was the 86.2% collapse from September 1929 to June 1932; the longest, the 60% plunge from March 1937 to April 1942. On average, bear markets lasted 19 months and dealt a 39% loss.
Whatever you do in the next few months—either buy or sell—will almost certainly have an enormous impact on your investment returns in the next 5-10 years.