Historic Equity Cycle

This is an interesting graphic provided by Jake over at econompicdata. The parameters of the equity cycle are surprisingly well defined, which leads me to believe that some sort of "irrational exuberance" is at play. I read somewhere that it typically takes around fifteen years for investors to "forget" about prior irrational decisions. An example of an irrational decision would be the equity bubble in the late 90's that paved the way for our current crisis. We are now descending the mountain (very quickly I might add) and will likely begin our climb into yet another irrational state. Could we potentially see the next bubble sometime in the 2020's?