When we look at stocks
One of the best ways to measure how expensive stock market has become is to use the Shiller PE Ratio. Price earnings ratio is based on average inflation-adjusted earnings from the previous 10 years, known as the Cyclically Adjusted PE Ratio (CAPE Ratio), Shiller PE Ratio, or PE 10.
As of March 2017, we are sitting at a CAPE Ratio of just under 29. From the below chart if you were to put your money in stocks for 10 years from this point you could expect to make(or lose) between approx. -2% to 6.5% on your money over that time. Also from the chart above typically the stock values get crushed rapidly just after these peaks.