I wrote some interesting speculation on this topic a few years ago. Roughly speaking, large premodern empires seem to consistently max out around roughly the same population size (~60M, with large-but-less-than-a-factor-of-two error bars). The few which manage to get larger than that through conquest rapidly split apart.
One generation. The Mongols were the only empire to get a lot bigger than 50-70M (they capped around 110M), and they promptly split in a war of succession.
I wrote some interesting speculation on this topic a few years ago. Roughly speaking, large premodern empires seem to consistently max out around roughly the same population size (~60M, with large-but-less-than-a-factor-of-two error bars). The few which manage to get larger than that through conquest rapidly split apart.
How rapidly?
One generation. The Mongols were the only empire to get a lot bigger than 50-70M (they capped around 110M), and they promptly split in a war of succession.