Yeah, it's just your own choice before the campaign. I always choose conquer 40 out of 60 + Eliminate all enemy clans. (so if there's 20 rebel regions left you win) That's realistic enough, and it's nice that you don't have to play through those last boring minutes and still get the nice 'end video'.

Btw, iirc, the victory conditions aren't choosable in orginial S:TW. Only in MI and WE versions.