I just finished my first run through the game, and it took me a whopping 63 hours.

I played slow and took the time to explore every little corner after the first level. I also played non-lethal for the vast majority of the game, and would knock out and hide most if not all hostiles on the maps before moving in on my final objective. Next I'll do a high action lethal run.

For me it took about 17 hours. I played on very hard and stealthy and non-lethal, but after the Boyle mission I stopped to explore and went for a more straightforward approach (but still stealthy and non-lethal). Barely knocked out anyone either, if I could sneak by without doing anything to the guard/person, I would.

9 hours. I just destroyed everything in my path. I regretted it when I saw the ending.

33 hours. I could tell I missed a lot by the end-of-mission screens but it was ok for future playthroughs. I was really into the story and having a great time. I played on Normal and went for non-lethal but at some point, I miswired a security system and two hostiles died. I just kept going with my plan and no one ever died by Corvo's hand. It was a great run and I'm looking forward to another try at a better non-lethal run; I wouldn't enjoy a high chaos run. Loved the ending I got.

about 35 hours non-lethal on normal

I thought I was slow at 28 hours. I played stealthy and did all the non-lethal missions, as well as a lot of exploring. I'm now doing a high chaos (but still stealthy) playthrough, and I think I'm taking even longer. This time I'm killing just about everyone so I can fully explore the maps and find everything.