>>519
Actually, the big ones seem to spin out about 10 or so generations in. To quote Sir John Glubb's _The Fate of Empires_:

Summary
As numerous points of interest have arisen in the course of this essay, I close with a brief summary, to refresh the reader’s mind.

(a) We do not learn from history because our studies are brief and prejudiced.
(b) In a surprising manner, 250 years emerges as the average length of national greatness.
(c) This average has not varied for 3,000 years. Does it represent ten generations?
(d) The stages of the rise and fall of great nations seem to be
The Age of Pioneers (outburst), The Age of Conquests, The Age of Commerce, The Age of Affluence, The Age of Intellect, The Age of Decadence.
(e) Decadence is marked by defensiveness, pessimism, materialism, frivolity, an influx of foreigners, the welfare state and a weakening of religion.
(f) Decadence is due to too long a period of wealth and power, selfishness, love of money and the loss of a sense of duty.