The short answer
A vintage mechanical watch should be judged as a vintage mechanical watch, not against a modern quartz watch or the clock on your phone.
A few seconds gained or lost over a day can be an excellent result, but there is no single figure that every vintage watch should achieve. Age, movement, condition, position, temperature and even how the watch is worn can all affect its rate.
What matters more is whether the movement is healthy, stable and performing appropriately for the watch.
From the bench
One of the tools we use at the bench is a timegrapher. It listens to the movement and gives us useful information including its rate, amplitude and beat error.
Those numbers tell a watchmaker a great deal about what is happening mechanically, but a timegrapher is not a crystal ball.
It measures the watch in a controlled position over a relatively short period. On your wrist, the same watch spends its day dial up, dial down, crown up, moving around, warming against your skin and resting overnight.
A watch showing an excellent rate on the machine can therefore behave slightly differently in everyday use. Equally, concentrating on the rate alone can hide other information about the health of the movement.
What we recommend
Give a vintage watch time to settle into your routine before judging its accuracy.
Wear it normally and observe it over several days rather than continually comparing it over a few hours. If its performance is reasonably consistent and appropriate for the movement, a small daily gain or loss is simply part of living with a mechanical watch.
A sudden or substantial change is different. If a watch that has been running consistently begins gaining or losing considerably more time, stops unexpectedly or develops a noticeably reduced power reserve, that can be worth investigating.
The aim isn't to make a fifty-year-old mechanical watch compete with an atomic clock.
It is to have a healthy vintage movement doing what it was beautifully engineered to do.