A brief timeline to illustrate how long each of the Cyrodiilic empires spanned throughout time. The Alessian Empire lasted the longest, spanning two millennia.
Even accounting for the Middle Dawn, it lasted several hundred years longer than the second and third empires, which can each be split into two distinct time periods.
By 4E 201, the third empire is a shadow of its former self and its future uncertain.
Further Reading
Pocket Guide to the Empire, 1st Edition: Cyrodiil