In a recent tweet, Jonathan Morin, Creative Director of Ubisoft Montreal, stated that Watch_Dogs will have about 100 hours of gameplay.
@vytla102 I would say the average player which does free roam a bit tend to reach the end within 35-40h. But doing everything is close 100h.
— Jonathan Morin (@Design_Cave) March 15, 2014
Whether that includes side missions or is merely an estimation for the hardcore gamer’s average free roaming, remains to be seen when the game is released in the second quarter of 2014.