From the Summary and Introduction: "In performing its analysis, CBO [Congressional Budget Office] made no assumptions about how long the occupation might last or about the size of the force that might be necessary. Instead, CBO's work focused on determining how large an occupation the U.S. military could sustain in Iraq indefinitely--while still maintaining acceptable levels of military readiness and not jeopardizing the quality of the all-volunteer force--under various policy options. Those options include using only combat troops from the Army's active component for the occupation, employing other existing U.S. ground forces as well, and expanding current forces to incorporate two additional Army divisions. CBO's analysis indicates that the active Army would be unable to sustain an occupation force of the present size beyond about March 2004 if it chose not to keep individual units deployed to Iraq for longer than one year without relief."
Congressional Budget Office: http://cbo.gov