Here are my rules for used/old watches:
A. If watch is 15+ seconds slower or faster with a lot of variation, get it serviced.
B. If watch is 15+ seconds slower or faster with little variation, get it regulated or do my own.
C. If the watch is vintage with a unique movement with hard to find parts, keep it serviced aggressively (maybe every 2-3 years).
D. If the watch has a common movement (ETA 7750, 2892, 2824, 2893, etc) and its keeping good time, wear it until A or B happens.
The parts on common ETA movements are inexpensive to replace and very easy to source, and will be for my entire lifetime.