if you want to go into statistics, normal average are useless for these things, as many empires last a few years, while others can last thousands. they don't fall in a normal distribution, you might need a geometric mean.

but also, empire is such a vague term. did the Roman empire fall around the 5 century? or do you count Byzantium as the Roman empire?

Did England start with the Norman invasion? or was it from before and the Normans were just a new dynasty?

it's something that's practically impossible to count, what's an empire? when it started/ended? and on top of that no normal distribution.