As with the previous DA games, I'd bet that the length of the game will depend in large part on which side-quests you choose to do, and how much time you spend exploring/examining/interacting with the areas, items, and both your Companions and the other NPCs you encounter. The "choices" you make along the main plot-line itself will not alter the overall story length, since the writers will strive to give every allowable path approximately equal complexity.
Again, based on the previous examples, I'd expect about a 60/40 split. One can chart a straight path through Ferelden or Kirkwall to the Final Battle, in about 50-60% of the "average" time it takes the average player; so about 20-24 hours. (For reference, I think the published record for DAO was around 35 minutes. But that was using game-glitches to skip major requirements, e.g. hopping almost immediately to the Landsmeet after recruiting Leliana.)
Most players will take more like 40-60 hours, and those of use who by nature strive for the "Completionist" achievement will take 100-plus hours. 