Forgive my oversight if this has been mentioned somewhere but I was wondering how many hours it might take to do a play through.

No word yet. Sorry.

As there are different ways to approach missions, different types of powers and builds for different playing styles, even an option to complete the game without killing anyone, it seems likely that at least the replay value will be high.

 
I was excited enough to do a calculation. I figured that this game would be heavy on textures and objects, light on map sizes and physical length of levels. That could be a gross misunderstanding of course. So my figure it based on a 3-hour perfect run-through done by an expert. But I also noticed that there are possible variations: to kill or not; to steal or not; to take on certain powers of not? (like bioshock?); to be stealthy or be seen. So let's say there are 2^4 (=16) variations to play. That's a good start, 3 hours perfect game, 16 variations.

Based on experience, your first time through takes about four times longer than a perfect run, if you're an explorer and a tinkerer, somebody who soaks in every detail, especially if it's an awesome game that catches your curiosity and has lots of interactible objects. So we could say your first run would be 12 hours, add another "perfect" run (unlikely but possible) at 3 hours and that's at least 15 hours of game play.

Now, when you get to trying every variation, you're not going to take as long with each variation as you would trying the last. Things in the game are constant and eventually it won't take you any time at all to try playing a different way, say, stealing everything and killing everybody crazily and out in the open and somehow surviving. Each time will take you a little less extra time (that four hours) to master. So to be liberal we could say three hours time 4 multiplied by (15 n-1 cumulative sum) over 16. Like 4 * (60/16 + 56/16 + 52/16 + 48/16).Which gives us 4*30, or, 120 hours if you played every possible way.

For me it's a great possibility that I'll buy this game. I would even have pre-ordered were it not for the lack of system requirements visible anyplace.

Whaaaaaaaat?

False. Where your targets are varies between playthroughs. Also what the various guards, thugs, etc. are doing varies.

Anyway, even a dry run would likely take longer than 3 hours. Do you think you could finish all of, say, Bioshock in 3 hours?

With a thing such as the chaos system, I believe the game will be fairly long. And it all depends on the way you play. They said that stealth will take a hell of a lot of time and I love stealth so that will be my choice.