Mightiest empires

Empire of Japan: minimum 1743 years to date
Byzantine Empire: 874 years (uninterrupted from 330 to 1204)
Holy Roman Empire: 844 years (962-1806)
Zhou Empire: 790 years (1046–256 BCE)
Ethiopian Empire: 666 years (1270-1936)
Khmer Empire: 629 years (802–1431)
Ottoman Empire: 624 years (1299-1923)
Roman Empire: 503 years (27 BCE-476)
Parthian Empire: 471 years (247 BCE-224)
Han Empire: 422 years (202 BCE-220)
Habsburg Empire (+ Austrian and Austro-Hungarian Empires): 392 years (1526-1918)
Mughal Empire: 331 years (1526-1857)
Ming Empire: 276 years (1368-1644)
Qing Empire: 268 years (1644-1912)
Russian Empire: 204 years (1721-1917)