I've always worked by the principal that if the watch starts gaining or losing time outside of it's normal operational levels, it's fine and just leave it alone. I guess if you really wanted to be accurate on that you'd get something to measure its beat rate. If it does start changing it at least needs a new oiling, and that's the time to service it. Obviously if something happens, like water gets into the watch, you have it serviced asap. That's my opinion anyway. I'm sure plenty will disagree .