We will withdraw when the job is finished. Period.
The job will never be "finished" so long as there are professional politicians and other parasites seeking to profit from government confiscation of assets and liberties.
Witness the "War on Poverty", "War on Drugs" and all the other endless "wars" on free citizens.
OK...well when is the job finished? Are the Iraqis unfit to govern and protect themselves? Is it also our "moral" duty to install democratic governments in Iran, Syria, and Saudi Arabia. At what point do we say that we have successfully brought the fight to Islamic terrorism, instead of having to fight it on our homeland? What is finished?