CAPE (aka price-to-ten-year-earnings, PE10[1]) helps devising a rule of thumb for timing.

Stocks don't deviate from their expected earnings for long, so when the ratio jumps over the median PE10, the market becomes expensive.

Holding cash during the period of high PE10 and investing it in stocks under low PE10 is a good idea for a person who's saving for retirement and doesn't want to become a professional investor.

[1] https://en.wikipedia.org/wiki/Cyclically_adjusted_price-to-e...