This is about how long my first no-kill run of Deus Ex: Human Revolution lasted, too. Well, sort of.
Not wishing to get too specific, your first combat scenario happens when you don't have cool robot parts, and you apparently need those to subdue foes in a non-lethal manner. Given the situation and the danger to your character and NPCs, the idea of not fighting back would be absurd.
That said, I tried to be a good cool robot person after that, and it lasted pretty late into the game.
There is an XP advantage to using non-lethal takedowns worth 20xp, which can add up. Granted, for that bonus to add up to a level on its own (5000xp), you'd have to literally take down 250 enemies that way, but still! There's good reason to leave them alive even if you're not a goody-two-shoes.