I have said before that we should as Greenspan ex-Fed said about the economy- take away the punch bowl before the party gets started.
There is a definite disadvantage in staying in Iraq too long- and there should be a small risk that Iraq won't be stable after we leave.
Leaving a year ago, there would have been a large risk or a near certainty that Iraq could spun out of control and pulled the entire region into chaos. Today that risk is less.
But I don't know if it's a small risk.
I say stay the course. We are somewhere in the middle of the surge. A surge implies we will reduce forces at some point. So when we start reducing forces we could continue reducing them to below the level they were before the surge started.
If this reduction causes serious instability, then we need figure out what the problems are. But essentially we should hope to use surge reduction as beginning of larger withdraw.

This doesn't mean I'm against any long term presence in the region- a small force for continuing training of Iraqi forces, perhaps. And/or enough forces that could allow a rapid increase of US forces if needed. This is something which requires political agreement. And of course any withdrawing of US forces will involve political discussions.
The parties involved should be of course the Iraqi govt, then Kurds and Turkey, and what Kuwait has to say. Maybe Iran.