Or rather, specifically, how long did it take to build the roman empire, well before it fell?
I'm looking for a ballpark figure, to work out the timeline of my story's history. Right now I have it at around 10-20 years (including the war that occurs in my timeline), given it was a hostile takeover.
Is this reasonable?
Rome started as a city-state kingdom in 753bc. And turned Republican in 509bc and at that time became expansionist. it reached its greatest territorial expansion in 117ad under Trajan, so Rome took 870 years to build, a smidge over your 10-20 year guess.
Or if we count from birth to collapse. Western Rome fell in 476ad and East Rome fell in 1453 so 1229 and 2206 years respectively.