In fairness to Keen he didn't say the bubble will burst right away .. he pointed to the Japanese property market and how their bubble slowly leaked out over a number of years... I think he said it's gone backwards for the last 14 years... by 40% or so.
but because he was misquoted (by some property spruiker) over the time frame he kindly conceded that he "lost" the bet - but he nevertheless has a "please ask me why I lost the bet" emblazoned on his t shirt to explain his side of things.