How long does a Macbook Air last?

Take care of it and keep it updated and you can reasonably expect 6-10 years of productive use.

You'll probably need to replace a battery in there at some point.

I've never retired any MacBook with less than 8 years of service, and I and my fam have had plenty in the past thirty years.

It could last decades but it will only be supported according to: Obtaining service for your Apple product after an expired warranty - Apple Support

Unsupported Macs do not receive app/OS/security updates.