Assuming you are not using guides and you want to collect all demons?
To complete a single playthrough with no guide or help, you'll take over 40 hours for sure, assuming you don't aim for 100%. For collecting all the demons, dude... it will take LOTS of time. I'm about 140 hours and still have many demons to unlock and fuse (mainly the Famed Race). --- If you're reading this, CONGRATULATIONS!! You're wasting your time!!