A watchmaker that goes by the handle of "Archer" on Watchuseek.com has posted an excellent summary to a question we also often get: what is the correct service interval for a watch? Unfortunately the answer is that it "depends" on such things as how often a watch is worn. That said, the normal recommended service interval is 2-5 years.
The biggest issue here is lubricating oils that breakdown, regardless of how a watch is actually worn, even if the watch is still keeping excellent time. Obviously if a watch suddenly starts slowing down or speeding up, this is a sign that some sort of service is likely. Other common problems that can be remedied via service: dry balance jewels, noticeable wear on the main plate from the winding pinion, and excess debris impeding movement.
The entire post contains many visual examples and is well worth reading.