Nobody knows exactly when the next update is coming but rumours point to a huge overhaul of the Air line and probably the introduction of a new 12" model. Once again THESE ARE RUMORS and as such MUST be taken with a BIG grain of salt.
As some users stated here in this forum, if Apple hasn't released the Air with Core-M for the Christmas Holidays, he might avoid Broadwell to go Skylake which would be a huge improvement in various aspects (performance, GPU, thunderbolt 3...), because Skylake will come for sure at 4Q 2015 (Intel ensured they wouldn't delay it despite Broadwell delays), so it's only 6 months from Broadwell to Skylake for the mid-high range computers & laptops.
Even if Apple goes Broadwell, they'll get Broadwell-U which won't come until Feb or so, so at least you should wait 4-5 months for a new MBA model.
IMO I was going to get the MBA Broadwell (if it comes out), but definitely gonna wait for Skylake.
the new model i think is for the spring event 2015 with broadwell
now apple will have a spring event because they will have what to talk about for at least 1-2 hours: watch available, 12" MBA, ipad pro and the 4k 21.5" imac