No Idea. But it will really depend on faction choice. If you take your sweet time the scripts for the 3 AI Romans will kick in and you will have to battle endless waves of stacks from them. If you are Roman yourself you will have to do the Civil War thing, though you can at least stunt the growth of your "allies" in the beginning.

I'd take the Britons or Scythia since they have OP units for lead battles whilst also being close enough to Rome to cull the 3 factions before they go in steroids rage war mode.
@ReluctantSamurai can give you details on when exactly they start churning out stacks of plastic men at you.