With last year's MBA updates it was about six weeks before they entered the refurbished section. Most Macs make it there in a few weeks though.
If by some miracle I want this year's model, I will probably wait and buy it via the refurbished section. They are for basically BRAND NEW. Let's say someone returned one because there was a dent in it. Apple puts a brand new complete exterior on it before selling refurbished. I have always had a great experience even though they're called "refurbished." My problem has always been waiting for them to show up in the refurbished section as I usually want my new Macs right when they're released.
I have to agree with Cheffy Dave though, that there is some serious speculation that they are using brand new ones and just repackaging them. It's like another market for those of us willing to wait, and accept the fact that it might be called refurbished but it looks and seems brand new. In addition, when checking build dates sometimes they seem newer than buying a brand new one as they're often two weeks old with even fewer batter cycles than reported new.
I would definitely recommend saving some money and waiting. In addition, the prior models are going to be available at gigantic discounts if Apple really includes 128GB Flash storage and 4GB RAM standard. Basically, the refurbished old models will need to be at the proper value vs. the new ones to sell. So if the new ones are $999 with 4GB RAM and 128GB SSD, I expect the old 11" MBAs with 2GB RAM and 64GB SSDs to sell around $729. When Apple sells prior generation MacBooks they're often listed at $729 to $769, and the current generation at $849 (and those were both $999 when new). So, Apple needs to mark these down accordingly, not just a percentage off what the price WAS, but a price that compares to the new ones minus the percentage discounts to account for them being old model, having less features, and etc.
I expect the current 13" MBAs with 2GB RAM and 128GB SSDs to be lowered to $999 or less in the refurbished section. The big problem for those wanting Nvidia GPU-based MBAs will be the lack of 4GB RAM in them in the refurbished section as I expect them to go quick whenever there are any available.