We were talking about empires.

Britain still exists as a polity, but the British Empire no longer does, so we say the British Empire lasted such-and-such number of years, because it’s already a thing of the past.

If we’re talking about how long political systems last, it’s going to get very abstract if we count different political systems as one just because they had the same capital (and sometimes not even that), or the same imperial core, or the same ethno-culturo-linguistic citizenry (or rather, first-class citizenry).

It gets increasingly difficult to define continuity if we start claiming the Roman Republic and the Roman Empire were actually the same political entity.