The Macbook Air was last updated on June 11, 2012. Recent history shows that Apple has been updating the MacBook Air every 9–12 months. That update cycle would lead to a March-June 2013 update range. The MacBook Air is still a major product for Apple, so look for them to follow the current update cycle.
One of the major new features obviously coming to the MacBook Air is a Retina Display. It is unlikely Apple can just drop the high-resolution display into the current Air, which suggests a major redesign coming as well.
It has also been widely rumored that a redesigned Mac Pro is also launching soon, so where does this put the MacBook Air launch? With Mac Pro rumored to release this spring, it is unlikely we will see a new MacBook Air arriving before Apple's World-Wide Developers Conference in June, as Apple likes to stage major updates.
Intel’s roadmap can also give us a clue to the MacBook Air release. Intel is planning to release their new processor series, code named “Haswell”, this spring. The mobile version of Haswell is likely to come out in June. This new processor will likely see improved battery life and processor performance, although that is up to some debate. Either way, Apple is likely to wait for Haswell before launching an all-new MacBook Air. It will take time for Intel to test and Apple to build up these new machines. Thus a June launch, with a July or early August release date is likely.
With a hardware redesign, it is difficult to tell what new features will be included. A Retina display is almost a given, as well as the new Haswell processor, but beyond those two features, the new MacBook Air becomes fuzzy. Will we see a new material like carbon fiber? Unlikely, as carbon fiber is an insulator which is not good for a computer that needs to get rid of its heat. Aluminum is good at this and will likely stay for the main material. Flash memory and storage options may change, but are likely to stay inline with the rest of the Apple lineup. Users have been wanting built-in LTE for a while now, but there is no evidence at this time to support that direction.
The MacBook Air is one of Apple’s best products. With a Retina display coming to the Air, a major redesign looks to be coming with summer. The Air was updated nine months ago and will likely see an announcement in the next 3–4 months, with Intel’s Haswell processor pushing the ship date out to June or July.