Short answer: basically a lifetime — if you service it.
A quartz watch has a lifespan. A well-kept automatic doesn't really. The r/Watches consensus is blunt:
"With regular maintenance, any mechanical movement will easily outlive you." — r/Watches
The key phrase is regular maintenance. A mechanical/automatic watch wants a service every 5–10 years — fresh lubrication and seals — and in return it runs for decades. Plenty of people are still wearing their grandfather's. As one r/BuyItForLife member put it, a mechanical watch "can last generations with proper maintenance."
Even affordable autos hold up: the community loves pointing out that budget Seikos routinely cross 10+ years of daily wear with no drama.
Buy a solid movement, wear it, service it on schedule, and "how long will it last" stops being a question. Our Carnival automatics run the workhorse Seiko NH36 — built to be that watch.
Good watches without the bullshit. — One Good Watch