How quickly could U.S. forces be withdrawn from Iraq? If you just moved them into Kuwait, it would take about three months, or more, depending on how much violent opposition the departing troops had to deal with. There are over a hundred American bases, and each one has to be shut down and emptied carefully to avoid actual or possible attacks. Moreover, there are many foreign civilians working for the American troops, and they have to be safely moved out, along with their belongings and equipment. While civilian security personnel can be used to provide protection, the primary responsibility is on American troops. Thus the total number of people being moved out could be over 250,000.
Another problem is that the United States is currently providing most of the logistic support for the Iraqi army, and to a lesser extent, the police force. The United States is still in the process of training and equipping the Iraqi security forces, so all of these programs would have to be handed over to civilian contractors (who would provide their own security.) It would take weeks, or months, for this to be completed. Another variable is how much military equipment would be given to the Iraqis, and thus would not have to be removed. Especially useful for the Iraqis would be the thousands of armored hummers and trucks. But arrangements would have to be made for maintenance and support.
Depending on when this withdrawal was done (next month, six months from now, next year), you would have to worry about the problems the Iraqi government would have maintaining order in Sunni Arab areas where the Americans were still providing most of the muscle for security. The withdrawal would be seen, by some Sunni Arab groups, as an opportunity to gather together several hundred gunmen and try to take over a town or neighborhood. The Iraqi security forces get stronger each month, but stretch them too thin, and the possibility of a civil war. It might not be a long civil war, but if the Shia and Kurdish militias get involved, the national government would see many of its security forces evaporate, making it just another player in the conflict.
Once in Kuwait, with its limited processing and shipping facilities, it could take over six months, and up to a year, to get all the heavy equipment cleaned (which it must be before getting on a ship) and sent off. This could be left to civilian contractors, if you wanted to get the troops home ASAP, for political purposes.
Some troops might be kept in Kuwait for a while, in case the political winds blew in another direction because of a civil war, or whatever, up north in Iraq. Militarily, there's no reason for a precipitous withdrawal from Iraq. But because of the misinformation, distortion and biased reporting about operations there, it's possible that sufficient political pressure could be mustered for force a rapid departure. The Department of Defense won't admit it, but the way these things work, there are almost certainly plans already filed away for how to carry out such a rapid departure. Then again, as a staff exercise, there's nothing exotic the military planning for getting out of Iraq in a hurry. What is difficult to plan for is the after effect of such a departure. Many bad things could happen, and that realization may be why the calls for a rapid departure have died down.