In his essay The Fate of Empires and Search for Survival Sir John Glubb (16 April 1897 – 17 March 1986) estimated the average duration at about 250 years. I am not in agreement with all his positions but the dates (from a quick check) seem about right (the full essay is available at: http://people.uncw.edu/kozloffm/glubb.pdf )

Dates of rise and fall and Duration in years

Assyria 859-612B.C. 247

Persia 538-330B.C. 208 (Cyrus and his descendants)

Greece 331-100B.C. 231(Alexander and his successors)

Roman Republic 260-27B.C.233

Roman Empire 27B.C.-A.D.180 207

Arab Empire A.D.634-880 246

Mameluke Empire 1250-1517 267

Ottoman Empire 1320-1570 250

Spain 1500-1750 250

Romanov Russia 1682-1916 234

Britain 1700-1950 250

According to his essay the US is now approaching that milestone.

I am not a historian and am therefore not qualified to comment further, there have been a lot more Empires in history (quite a few in India alone for example) so this selection may not be truly representative