The person or people running the DOOM Twitter page made this claim a couple days ago. Difficulty dependent, of course, and I also wouldn't be surprised if this is how long it will take to 100% the SP portion.
https://twitter.com/DOOM/status/697883390821982208
Send me to hell with a piss filter if old.
The difficulty level plays a factor, but folks playing at the office average 13+ hours in the campaign
https://twitter.com/DOOM/status/697883390821982208
Send me to hell with a piss filter if old.