Imagine asking FDR in 1942 how much its going to cost, and how long are we going to be over there...
The answer then, as now, is that its unknowable. It depends in part on the enemy. How long will it be before the enemy sees this fight as hopeless, unwinnable, and gives it up? An important piece of this puzzle is the perception the enemy has of our resolve. If he sees us waver every time we take a hit, then hope for him is still alive, and the fight continues.
I would say that the time table is tied to the Iraqi government's ability to defend itself, and there are probably still a couple of names from the "deck of cards" that we want to take down before we declare victory. Those two are probably the bottom line issues, and how that relates to a calendar is probably irrelevant.
Beyond that, my guess is that we aren't leaving until Syria has been pushed into holding elections. So my guess is we're there for another five years.
You raise a good point. That is why the military [draftees included] had to serve for 'the duration' plus 6 months. The metric was unconditional surrender.