How long will Apple support updates for a 2015 MacBook Air?

Depends on the OS you are running.

Apple has traditionally supported the previous two OS versions with security updates.

Your Mac can run Big Sur, so if it cannot run the next major OS, you would get OS security updates for at least two and a half years more.

But, that is all dependent on what happens with the next OS and whether your Mac can run on it.

Dougp27 wrote:

It has theMac OS XVIO.11 El Capitan if this info helps.

Again, still no way to know.

MacBook Air Early 2015 is supported with Big Sur; the current macOS release.

You’re on an older release. Upgrades to more recent releases are available.

Hardware support (parts and maintenance) usually gets five to (in some places) seven years.

Obtaining service for your Apple product after an expired warranty - Apple Support

The next-older MacBook Air model from what you have is vintage.

Specific details of when this McBook might transition to vintage or eventually to obsolete status are not known.

How long this MacBook Air will continue to get macOS software updates? Unknown.

Apple is also transitioning architectures from Intel x86-64 to Apple silicon Arm architecture processors, too.

The current model MacBook Air M1 2020 is based on an Apple silicon M1 processor. It has transitioned to Arm.

There's no way to know. It can run Big Sur and Big Sur should be supported for approximately 3 years.

I’m looking at purchasing one that is an I5 processor, 8 mgs of Ram and 256 SSD.

How does that alter what I stated?

You can install Big Sur on that Mac, but El Capitan is no longer supported.