The theoretical answer
Indefinitely. A mechanical movement is a set of metal parts that can be cleaned, re-lubricated, and have worn components replaced. There is no process that makes it unrepairable, which is why hundred-year-old pocket watches run accurately today.
Every other durability claim on this site is a claim about a material. This one is a claim about an economic choice.
The practical threshold
A full service from an independent watchmaker typically costs 150 to 400 dollars, more for an in-house or chronograph caliber.
Below roughly 400 dollars of watch, that service costs more than a replacement. So cheap automatics get run until they stop and are then replaced, whatever the movement was theoretically capable of. Above that line, servicing is rational, so the watch gets serviced, so it survives.
That is why a Seiko 5 is a ten-year watch in practice and a Hamilton Khaki Field is a fifty-year one, despite both being perfectly rebuildable. The arithmetic differs, not the engineering.
Service intervals, honestly
The traditional five-to-seven-year advice dates from older lubricants that degraded faster. Modern synthetic oils last considerably longer, and plenty of watches run within specification for a decade or more.
Better indicators than a calendar: accuracy drifting away from where it used to sit, the power reserve shortening noticeably, or the watch sounding rough when you wind it. Those mean service it now. A watch that is keeping good time probably does not need anything.
What actually ends watches
Water ingress. Gaskets perish with age and heat. A water-rated watch is rated as it left the factory; keeping that rating means replacing seals every few years. Most water-damaged watches were rated 100m and had original gaskets from a decade earlier.
Impact. A hard knock can shift or break a balance staff. The G-Shock exists because of exactly this.
Corrosion. Sweat under the lugs and inside the bracelet links, over years. Rinsing a watch occasionally genuinely helps.
Loss. A real and large category. Spring bars fatigue and fail, and a watch that comes off a wrist in a car park is gone. Replacing spring bars every few years costs almost nothing and prevents the most common total loss.
Quartz, and the G-Shock exception
A quartz module is replaced rather than repaired, which sounds like the opposite of buy-it-for-life. In practice it is often better: modules are cheap, straps and bezels are catalog parts, and the resin case is close to indestructible.
Thirty-year-old DW5600s are in daily use everywhere. Cheap replaceable parts beat expensive serviceable ones more often than watch enthusiasts care to admit, and that is a general principle on this site rather than a quirk of one product.
