Without a doubt, every piece of information out there shows that if you want to be a successful investor, you need to invest for the long-term, with an outlook stretching over 10 or 20 years at the very least -- especially if you are a passive index investor.
Investopedia notes, since the 1920s, "individuals have never lost money investing in the S&P 500 for a 20-year time period." This includes setbacks such as the Great Depression, Black Monday, the tech bubble and financial crisis. If you outside the US the results are even more compelling.

The Credit Suisse Global Investment Returns Yearbook...

