"As to volatility, we know that a 10% correction comes around every once each 2 years, and that we can count on three corrections of 5% over that same time period. We cannot predict these, and have yet to find anyone else who can. We simply accept this as a part of investing, and compose our portfolios with that in mind We also try to make sure that emotionally, we are prepared for these inevitable hiccups." -- Barry Ritholtz

"Over the last 100 years the stock market has experienced on average a 5% correction three times per year, a 10% correction once per year and a 20% correction once every three and a half years.

"Stock market investors should expect to lose a little money quite often, see a correction occasionally, lose a decent amount every couple years and lose a lot of money on an Olympics-like schedule.
5% losses three times a year.
10% losses once a year.
15% losses once every two years.
20% losses once every three to four years
This realization that you know something is eventually going to happen, but you have no control over when or why it will happen can be extremely liberating as an investor. Understanding what you do and don’t know is a huge step in the right direction."
awealthofcommonsense.com - How market crashes happen