3500-2340 BC: Sumerian Civilization = 1160 years
2340-1900 BC: Akkadian Empire = 440 years
1900-1100 BC: Babylonian Empire = 800 years
1100-612 BC: Assyrian Empire = 488 years
27 BC- AD 312: The Roman Empire = 339 years
AD 312-1453 The Byzantine Empire = 1141 years
1350-1918: Ottoman Empire = 568 years
1368-1644: Ming dynasty = 276 years
300-1300: Mayan civilization = 1100 years
Looking at most of these cases, and seeing that our American civilization is only 226 years old, I'd say we have a little ways to go before our empire crumbles.
Thanks for the rundown. I think some other posters pointed to more recent 'empires' as more short lived. History has speeded up - faster communication? faster travel? faster living?
Failure to reproduce may the the suicide belt of our Empire, without outside challengers.