I know that there are a bunch of different ending's and different ways the game can play out, but can anyone give me a general idea on how long this game takes to play threw?

About 15 hours on my first playthrough.

15-20 hours if you do every mission and read all emails and related preparation. Could be shorter if you just go straight for the main missions but I suspect that can change how the story goes too without all that extra intel and such.

My first playthrough was almost exactly 20 hours, but I was playing it stealthily without specializing stealth (which I really enjoyed btw), so that probably accounts for 4 or 5 of those hours I'd guess.
 
I didn't want to dive straight back in, but it definitely seems like there is a ton of replay value.  The story branches far more than either Mass Effect game for example.  Throughout the game you might ally with certain characters and spend hours in their company, yet it's also possible to hardly interact with that person at all and end up allied with other people entirely.
 
You can also have fun counting the number of roles Nolan North voices.  I think I counted at least 3 seperate characters.

The game felt mad short to me, I beat it within a normal rental period, which was dissapointing.  
 
But there could be some replay value. 

I got about 3 different endings, just doing different things in the last mission,  i'm sure there might be more if you do things differently earlier in the game.