I think that's far too long to keep this going on, and 18 months in particular would have the U.S. attacking in the summer heat. Not a good scenario for soldiers in MOPP gear. It would be a disaster.
Also, it's unimaginable that the sort of political tension this is causing could be sustained for such a long period. Neither the American people nor Middle East countries would tolerate such a protracted "crisis".
Everything I've seen so far points to late January or early February of 2003 for commencing attacks, with January a clear favorite for the beginning of the traditional bombing campaign.
Of course, we'll know soon enough if I'm right or wrong.