According to creative director Jonathan Morin, Watch Dogs will take around 35-40 hours to complete the main story, and around 100 hours to complete the game 100%.
Here's the tweet:
@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
Watch Dogs is scheduled to launch May 27, 2014 for the PlayStation 3, Xbox 360, PlayStation 4, Xbox One, PC and Wii U.