The US never said it knew where the weapons were. It was not up to the US to provide proof that Iraq had weapons...It was up to The Iraqi government to proof that they didn't...and they never (after 12 years) gave any solid proof that they're weapons had been destroyed.

The US wouldn't give the UN weapons inspectors the time to find the weapons - but now that the US are in there, now they're giving themselves all the time they want.
Face it the UN inspectors couldn't find sand in a desert. We can take all the time we need because now there isn't an Iraqi government that will kick us out if we get too close.

It may take a year for 300,000 troops and 1,000 scientists to find the weapons...especially if the iraqi scientists don't cooperate. Hell, they're trying to search a country that is 171,599 square miles. And it isn't like they'll be sitting in the open desert either...They'll probably be in underground bunkers or storage facilities or just buried somewhere in the desert..some of locations probably aren't known to anyone except, Saddams regime.