Well, considering that I still have the old LC475 that I got in the mid-90's ...
(and yes, it still works, although TBH it's not useful for very much anymore. I still keep my old writing from that era stored on its HDD though.)
When I bought my near-base model 2012 MBA (128GB, i5, 8GB RAM) I was anticipating keeping it at least as long as I've had that ancient LC475. However, I'm already drooling over the 2013's (12hr battery life??), so I might be selling this one in a year or so (while it still has some AppleCare left) to help subsidize an upgrade to a 2014.
However, as a few other posters have already said, a lot depends on the $$$ situation. If I wait until I "need" a new computer, I'll probably be waiting a LOT longer than just another year, lol ... Since this one works just fine and will probably continue to do so for several years, a new computer is lower on the priority list than several other items I'm considering.