It really depends on playing style, if you blitz a lot you can win very fast as Sweboz or Baktria, perhaps also as Aedui and Arverni if you can raid Rome early on.
I guess it will take longest for the steppe factions and perhaps for the Romani. The Romans have one of the easiest military and economic situations in the game, but also need to conquer and raid a lot to win.

In my games I mostly ignore the victory conditions for roleplaying and other reasons, like waiting for reforms or such, so I can't really tell. As Baktria for example I often wait very long before I destroy the Saka or expand too far westwards.