To be honest, the thread title is a little misleading; it asks about the longevity of the MBA, whereas, reading the thread itself, it is obvious that what the OP is asking about is for how long people decide to keep their MBAs.
In general, I decide to keep mine for between two and a half and three years; Applecare is what really determines the lifespan of my computers, not technological breakthroughs. My first Apple was a MBP, which I bought in April 2008, a belated birthday present to myself; in September 2008, after two years abroad, I bought a MBA, - the old 'Rev C' and early the following year, sold the MBP. Both had Applecare.
The MBA, in turn, was gifted to my brother last summer, and I replaced it with a maxed out 13" MBA (8 GB RAM/512 SSD and Core i7), the computer I am writing this on. As that was CTO, it took longer than expected to be ready, - it wasn't ready when I was due to return abroad - and, as I work abroad, I had need of a computer to take back with me.
Thus, as a stop gap, last summer, I bought a 2012 11" MBA (refurb; 8GB RAM, 256 SSD, and strangely, Core i7); a fantastic little machine, (to which - as with the CTO) I also added Applecare. A colleague lusted after the 11", and two months ago, I sold it to him.
Having said all that, the 2013 MBA is a superb machine, well worth getting, and the battery life alone makes it an awful lot better than the model offered in 2012 (which I also had, but which I subsequently sold).