I replace my MBA every year. However, I always pass my one year old machine to a family member. So... I typically buy them with a 5-6 year expected lifespan for each computer. I just personally get a new one every year (since I pay).
It is a great way to rationalize a technology addiction. It applies to iPhones, iPads, and iMacs too!
/Jim
Between 3-5 years. I get irrationally attached to my machines and enjoy using them long after they become dated. I'm typing this on my late 2007 BlackBook that I still use for basic internet browsing and Netflix (for fun at home).