Good question, Hans, and I agree there will not be a universal answer. Here are a couple of my thoughts:
Let's say you reach a five year interval and are trying to decide on a service.
Reasons to do so:
- Timekeeping is not as good as what you enjoyed in the past
- Watch operation has changed (lower PR, noisy automatic winding, changed feel for manual winding)
- Watch is used in tough environments (diving, etc) and seals need to be checked/replaced.
- Watch is vintage and parts are no longer available.
Reasons not to service:
- Keeps time well
- Operation, feel, and sounds haven't changed.
-Watch hasn't been abused.
- Watch service and parts are available.
My perception might be wrong, be it seems to me that the costs of service plus replacement of worn parts doesn't exceed the basic service by enough to make it worthwhile doing a lot of pre-emptive maintenace. Rolex, for example, charges about $550 for an overhaul of the watch. Let's say you service every 4 years, and as a result, all that Rolex ever needs to do is clean, oil, and adjust. After 20 years, you are out quite a bit of money. On the other hand, let's say you went the entire 20 years without a service, (as a number of Rolex owners seem to do!), and the service after that 20 years requires replacement of a number of worn or out-of-spec parts. Probably more expensive, but probably not enough more to make preventive maintenance cheaper. Plus, even in spite of the neglect, your watch after repair is as good as new.
I think the other factor is the ever-decreasing number of skilled, independent watchmakers who can service locally and offer good turnaround times. I suspect that in the future, factory or OEM servicing will only increase in prevalence, both for the reason above and because of the perfectionist bent of most mechanical watch enthusiasts/collectors. The in-house servicing of most brands tends to involve high prices, shipping, and long turnaround, but offers essentially complete restoration to factory spec. For example, Omega Bienne serviced my 1959 Seamaster last year to the tune of $700, but replaced about half the movement (based on the little baggie of parts they sent back to me) and returned the watch running beautifully. Had the Seamaster not required such extensive work, it still would have been $700!
Analogies to car maintenance abound, but I'm not so sure the economics match. Changing the oil in my car costs about $22, while an engine rebuild following ignored oil changes could be over 100x that. If a skilled clean/oil/adjust were really $10 and done in a day, preventive maintenance of watches would be a no-brainer.
Tom