I "found" my gown in January of 2002, and made the deposit about a month later. It was custom though and we bagan building it in Nov. 2002... I was married July of 2003.
It depends on the order load of the manufacturer... Maggie Sottero dresses can take 8-9 months in heavy wedding season... and then you need time for alterations. Check with the bridal shops and give yourself at least a month of extra leway just in case.