Ever since I played Human Revolution I found myself wanting to see more and more. I beat it twice still doing my third playthrough, I have just about all of the achievements for the game. Then during all of that I started playing the original game on my PC, and there went 2 or 3 weeks of my life. So having combined all of that I would say that both Deus Ex games have taken 6 or 7 weeks of my life.
Now I need to get back to playing the new DLC.