Based on this measure, the value of the stock market is near a historic high.
The last two times the market got to this point were before the tech bubble implosion (2000-2002) and the global financial crisis (2008-2009).
History also shows that 10yr forward equity returns have a big negative correlation to the proportion of the stock market to GDP. Current levels imply a forward anualized return of about 3%. Not a compelling reason to hold stocks right now. Indeed, this is just another indicator showing the 'everything bubble' is nearing its demise.