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-01193.
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.