We've got a preview of Hunted: The Demon's Forge coming up on the site (in fact it may already be here by the time you read this).
In it, our Svend picks up the following useful bit of info:
"Bethesda tells me it will take a 'competent gamer' about 20 hours to finish the campaign..."
That's the single-player of course.
Guess what, that preview can be read right
here, right now.