You can't predict a time frame on things like this - it could swing either way. Look at Afghanistan's history - they've had people invading and then leaving for centuries. I can always see some form of military presense there. It might not even end in our lifetime.
As for Iraq, it could be the tip of the iceberg - we could be in for many more years to come and for as long as there's insurgency, there will be troops needed. I can't see the insurgents slowing down. A bleak view but realistic nonetheless.