A recent Tweet by Jonathan Morin, the Creative Director on Watch Dogs, claims that the main story should take the average player between 35-40 hours to complete.

He goes on to say that for 100% game completion, that time is closer to 100 hours of playing.

Keep in mind that developers tend to oversell completion estimates prior to a game's release. But even if you cut those times in half, that is still a fairly lengthy experience.