Air tags Life Expectancy

I’ve never had one fail. I have replaced the batteries in them.

You may want to look at this.

Some batteries are coated with a buttering agent to make them taste bad in an attempt to keep children from putting them in their mouths. This can cause problems with AirTags as well as some other products. You should not use batteries coated with bittering agents as they may not work. That might explain why they’re not working after you replaced the batteries.

Sorry, but no one here (we are just Apple-product users) can answer your question on the lifespan of an AirTag. At best we only know what Apple has provided all of us and that is the battery life (not the device itself) typically will last a year.

My best guess would be the environmental or physical conditions that the particular AirTag is subject to, will determine its useful life.