For practical purposes, the financial memory should be assumed to last, at a maximum, no more than 20 years.

A Short History of Financial Euphoria by John Kenneth Galbraith.  Page 87.

Let it be emphasized once more, and especially to anyone inclined to a personally rewarding skepticism in these matters: for practical purposes, the financial memory should be assumed to last, at a maximum, no more than 20 years. This is normally the time it takes for the recollection of one disaster to be erased and for some variant on previous dementia to come forward to capture the financial mind. It is also the time generally required for a new generation to enter the scene, impressed, as had been its predecessors, with its own innovative genius. Thus impressed, it becomes bemused by the two further influences operating in this world that are greatly seductive of error. The first, as sufficiently noted, is the ease with which any individual, on becoming affluent, attributes his good fortune to his own superior acumen. And there is the companion tendency of the many who live in more modest circumstances to presume an exceptional mental aptitude in those who, however evanescently, are identified with wealth. Only in the financial world is there such an efficient design for concealing what, with the passage of time, will be revealed as self- and general delusion.