Honestly, no matter how Apple markets how efficient OSX is, you will eventually kick yourself if you brought the bare 2GB RAM.
Here's my take:
11.6 MBA 1.4/1.6 2GB (2 years max is probably what most of us macrumors people will get out of the people).
11.6 MBA 1.4/1.6 4GB (you could stretch it out to 3 years).
13.3 MBA 1.83/2.13 2GB (again probably closely to 2 years maybe 2.5 years)
13.3 MBA 1.83/2.13 4GB (3 years, maybe stretch it to 4 years).
Not sure those with 2GB RAM will be able to smoothly support OSX 10.7 (Lion)
I have the 13.3 MBA 1.83/4GB and honestly am looking at 2 years use for my own use. Maybe I will hand me down to a family member. But that's how long I plan on using my MBA.
I don't keep computers that long. My 2007 24 inch intel core 2 duo iMac lasted me 2 years before I resold it. My 2008 Macbook 2.4 intel core 2 duo penryn lasted 1 year before I resold it.
I still have my 2009 iMac 27 inch core i7. I will probably end up keeping that 3 years and give it to my mom as a hand me down.
I kept my 32GB iPad 3g for about 7 months.