The speed with which an occupying military force leaves a country generally depends on the degree of urgency...

For instance, the Germans left France (unwillingly) in quite a brief period. They had no choice.

The last Americans in Saigon when the city fell left about as fast as they bloody well could when it came to it.

My point being: an army can leave slowly...over several years...or quickly...in a few months, a few days, or even in some cases in a matter of 24 hours. It just depends on the sense of urgency, that's all.

I would think that a sensible period of time for withdrawing US troops from Iraq under most forseeable conditions would be about a year to a year and a half. Maybe 6 months if they really pushed it.

But you see...the bosses don't want them to leave Iraq. Invaders don't usually leave unless they are...

1. kicked out by superior force (as the Germans in France)

or...

2. they find it's simply too unprofitable to stay (as the USA in Vietnam).