The last time they redesigned the MBA (2010) I believe it went on sale immediately and my local Apple Store was sold out immediately. I was able to get one about 3 days after the announcement if I remember correctly.
Despite rumors earlier this week suggesting the 12-inch Retina MacBook Air could see a launch during Apple's upcoming March 9 media event, The Wall Street Journal tonight released a report indicating the notebook won't be shipping until Q2 2015.