The Schiller PE is at a rare peak. Historically, when it is over 25, the following 10-year real return for stocks has been under 1%. Pretty grim if this is the first 10 years of your retirement. I may wait until it is under 15 (real stocks have returned >10% over 10 years historically in such settings).
There has been a lot of discussion on the Shiller PE recently and different people have different views on it. I guess I would point out this:
I don't dispute a low Shiller PE,
if you can catch it or if it occurs, has been correlated with higher future returns, but are you factoring in the opportunity cost of waiting...for maybe as long as 2 decades?
It briefly came down to ~15 in '09 as a result of the financial crisis...if one had bought then you'd do very, very well, but it was also a disproportionately large recession compared to normal, and also a very hard time to commit all your capital for many people. It also only briefly touched around 15 before rising again (short window). Before that, it hadn't been under 15 since the late 1980's. You may be waiting a really long time.
I haven't run the numbers but my gut tells me someone that invested in the early 90s with a Shiller PE at 20 (after 'missing' the last sub-15 window) and then collected 15+ years of dividends and compounding growth was still better off than someone who held out the until they could buy in 2009. The S&P 500 still went up 70% from 1992 to the very bottom of the stock market collapse in 2009 even before considering dividends.