In terms of how far the market can run before reality and gravity finally reassert themselves, bursting the centrally planned bubble and prompting a 2008-style "correction", Grantham defined a "full-fledged" bubble as S&P 2250 and warned that a retracement of some 50% was possible depending on how assertive the Fed's response to its real favorite economic indicator (stocks) turns out to be. 

In GMO's latest quarterly letter, Grantham is out reiterating his view that although US stocks may not have reached their peak in what he accurately calls a "strange, manipulated world" (we prefer "new paranormal"), he's sticking with the idea that "bubble territory" is likely just around the corner as the Yellen Fed is "bound and determined" to facilitate and inexorable rise in asset prices. He also notes that the Yellen seems no more inclined than her predecessor to take Jeremy Stein's advice on being careful not to adopt an "implicit policy of inaction" when it comes to bubbles. Here's more:

The key point here is that in our strange, manipulated world, as long as the Fed is on the side of a strong market there is considerable hope for the bulls. In the Greenspan/ Bernanke/Yellen Era, the Fed historically did not stop its asset price pushing until fully- fledged bubbles had occurred, as they did in U.S. growth stocks in 2000 and in U.S. housing in 2006.