According to the games creative director, Watch Dogs length will be 35-40 hours on average.
This comes via creative director for Watch Dogs, Jonathan Morin’s Twitter account where a fan asked how long the game would take to be completed. Morin explained that the players that do a little bit of free roam during the main campaign will finish in about 35-40 hours. He also mentioned that if you’re the type of player that does almost everything then it will take you close to 100 hours.
@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
After an unexpected delay last year Watch Dogs is coming out on May 27, and will be available for Xbox 360, PS3, Xbox One and PS4.