The short answer
There isn't a single service interval that makes sense for every vintage watch.
You will often hear recommendations such as every three, five or seven years, but the reality is more nuanced. How the watch is used, the movement inside it, the quality of its previous servicing and its present condition all matter.
With a vintage watch, we are generally more interested in how the movement is behaving than simply how many years have passed since somebody last worked on it.
From the bench
A mechanical movement relies on very small quantities of lubricant applied to equally small working surfaces.
Those lubricants don't last forever. Over time they can deteriorate, migrate or dry, increasing friction and eventually contributing to wear.
But that doesn't mean every movement reaches the same point on the same birthday.
A watch worn every day has a very different life from one worn occasionally. Two examples of the same calibre may also have completely different service histories stretching back fifty years.
This is why the bench matters.
Rate is useful, but it isn't the whole story. We can also look at amplitude, beat error, power reserve and consistency, and listen to what the movement is telling us.
A watch keeping apparently excellent time isn't automatically proof that everything inside it is healthy. Regulation can alter the rate; it can't replace deteriorated lubrication or repair wear.
Equally, opening and dismantling a perfectly healthy vintage watch unnecessarily isn't something we advocate simply because a calendar says a particular number of years has elapsed.
What we recommend
If you know when your vintage watch was properly serviced, keep a record of it. That's one of the reasons service history is useful.
Then pay attention to the watch.
A noticeable change in accuracy, reduced power reserve, difficulty winding or setting, intermittent stopping or a significant change in how the movement sounds can all justify having it inspected.
If you've bought a vintage watch with no reliable service history, an inspection is much more useful than assuming that because it runs, it must recently have been serviced.
And if a watchmaker who knows the movement recommends servicing it after inspection, that's a much better reason to do the work than simply reaching an arbitrary date on the calendar.
Service history gives us a date. The movement tells us when it needs attention.