Watch Dogs will take most around 35-40 hours to complete with completionists taking close to 100 hours if game creative director Jonathan Morin is a believable source. Which he is.
A fan on Twitter asked the director to give up the details on story length and Morin was nice enough to answer.
He later went on to explain this does not include the online stuff nor the variety of random pointless things one can do while traversing Chicago. He also was very clear that he had not done a speed run so there is no speed run time on file yet.
Watch Dogs has officially released a launch date of May 27th on Xbox One, PS4, Xbox 360, PS3, and PC with the Wii U coming later this year.