Act one goes from the start of the game to the Winter city. but when does act 2 end, and how many acts are there going to be? Is act 1 a third of the story? A quarter?
You can see a preliminary list of planned regions for the game here. Savin has mentioned that some details have changed in the intervening years but that should give you a rough idea where things are going. As Aly pointed out while I was in the process of typing this, the story is in three acts.