Manufacturers give different intervals, and they have changed over time. Many modern makers suggest a service roughly every five to ten years. Older watches with traditional oils often need one sooner. There is no single correct figure, and a manufacturer's own recommendation takes priority for its watches.
Signs a service is due: timekeeping that changes noticeably, a shorter power reserve, rough or gritty winding, stopping when worn, a date or chronograph that sticks, or moisture under the crystal. Moisture needs attention straight away.
Why it matters: lubricants thicken and migrate over time. Once they dry out, metal wears against metal and pivots and jewels are damaged. A watch that is still running is not proof that it is healthy.
What a full service involves: complete dismantling, cleaning, inspection and replacement of worn parts, reassembly with the correct oils in the correct places, then timing and regulation. Water-resistant watches should also have their seals and a pressure test. A 'quick oil' without dismantling is not a service.
Vintage and antique watches: when parts are replaced, the watchmaker should record which ones and keep the originals where possible. Ask for this in writing.