Ubisoft have stated that the main campaign in Watch Dogs should last you roughly between 35 to 40 hours.
The length was revealed by creative director Jonathan Morin who stated on twitter:
@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
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