Common advice says service every five years, but service is expensive and some people run watches much longer. What observable signs actually indicate a mechanical watch needs attention, as opposed to just being due on a calendar?
Discussion record: RWF-07745.
Clarifying question first: do you have a baseline? If you've never recorded the watch's rate or power reserve, 'drifting' is hard to judge. Start a simple log now with daily offset against a reference time for a week, and the hours it runs from full wind. Once you have that, later changes mean something.
The calendar interval is a conservative default, not a rule. Useful signs, in rough order of seriousness: power reserve noticeably shorter than spec from full wind, rate drifting outside its usual range, the watch stopping on the wrist, gritty or rough winding, and the date changing slowly or late.
If the manufacturer provides a suitable procedure, a running-time log can support a service enquiry. State the winding method and stop time. A shortened run does not identify the cause by itself.
A sudden rate change has several possible causes. Ask a watchmaker to test suspected magnetism rather than diagnosing it from a compass or attempting an adjustment based on forum replies.