For what its worth, one of the historical indicators a lot of people quote is the Schiller PE Ratio.
http://www.multpl.com/shiller-pe/The theory is companies are worth an underlying multiple of their earnings.
The ratio has been more challenging to benchmark since 1995, we've had a lot of technology companies that are public without any earnings and we're still working through some huge negative earnings/overstated losses from 2009-2010 in the banking sector. (the banking sector stuff is a much longer explanation)
There's also a higher percentage of the population invested today than through the 70s/80s with the prevalence of technology and in the depths of 2009 we barely got to the 100 year median/mean.
I think come 2020/2021 this will be a good measure again.
Other than that, its entirely up to you:
- Lots of people said tech stocks were on sale in 2001 and got beat over the head for two more years
- Alternatively, in 1987 a lot of people experienced a 27% drop, sold out, and didn't participate in the huge bull market that ensued for the next four years.
"The four most expensive words in the English language are "this time it’s different." " - John Templeton