In the past three millennia of world history, the average imperial power has survived for 215 years. America, measured from the ratification of the constitution, is now 223 years old.
I find this hilarious, because it sounds like one of those quaint, patently ridiculous "facts" that people pass around online, but I found it in a scholarly article by a guy who researched the fact himself. Evidence:
"Using a data set of empires that spans over 3,000 years, I wanted to create a model that could show us, statistically, what the lifetimes of empires look like. There are many more complex and intricate models of how civilizations grow and decay, but perhaps something could be gained by creating a very simple model that looks only at life span.
This data set is expansive, including everything from the Babylonian Empire of ancient Mesopotomia - known for such contributions as Hammurabi’s Code - to the Byzantine Empire, which has provided us with the eponymous word for red tape. Some of the world’s empires lasted an exceptionally long time: The ancient, and now little known, Elam empire located in present-day Iran lasted a thousand years. Others were short-lived, for all their power: The Phrygian and Lydian empires were around for only about six decades each. (The data set, based on earlier research in empires, ends at 600 A.D.)
If you crunch these all together, the first thing you discover is that the average lifetime of these powers is 215 years.
If you’re playing at home, this number is pessimistically eerie: It’s been 223 years since the ratification of the US Constitution. And that should perhaps give us some pause. To make this explicit, the United States has now outlasted the majority of the empires in my historical data set, and is now crossing the threshold into hoary old age."
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http://www.spaulforrest.com/2011/11/how-long-will-america-last.html
