Please consider donating to help offset our high running costs.
Very useful, thank you. 😉
First, why don'y you believe what the seller told you?
Second you are ignorant enough to think the forumer is joking whe he is telling you absolutely the truth. A truth easily available in every watch forum if you have taken the time for search as there are thousands of threads like this. Respect to be respected.
OP states explicitly that he is merely requesting an objective criterium for need of service. Nowhere is he implying a lack of trust in the seller. The answer that "the watch will tell you..." is hardly informative. Neither is your post, and then you have the nerve to call him ignorant. We don't play that game here.
OP called the other forumer "a dick"
Besides FYI, an ignorant is someone who ignores something. In a way or another we all are.

I don't know what you play or not. Take the time to read all posts. OP called the other forumer "a dick"..Besides FYI, an ignorant is someone who ignores something. In a way or another we all are.
Until you earn some “chops” around here, moderators take note of newbies
There is no one answer to this question, because it will depend on many different factors...
The first one will depend entirely on how one defines "needing service" as this can have very different meanings to different people. For some, if the watch is running, it doesn't need service. For others, if it runs but doesn't keep time properly, it needs service. For me, it needs service when it's at a point where damage is being done to parts inside, and watches are not effective communicators in telling you when that is happening.
Second, if the watch is modern or vintage, and who you intend to use for servicing can be a factor. For example a modern watch where parts are readily available, and you intend to use the brand service center - in that case run it until it dies if you like, and it probably won't cost you any more than if you had done the service sooner. The only thing to keep in mind should you choose a path like this, is that water resistance is not a permanent feature, and requires maintenance to stay intact. The last thing you want is for your watch to tell you it needs service like this:
Water intrusion that damages parts like dials, hands, any some movement parts is not typically covered in a standard service fee, so if you get your watch wet regularly, then waiting until it tells you it needs service may not be a smart strategy.
If you have a vintage watch and the parts for it are discontinued, then letting the watch go until there's a problem is not generally a good idea. The parts can be very expensive to replace if you have to find them on the open market. For watches like this, more frequent servicing that preserves the parts that are in the watch is likely a better strategy, and will save you money in the long run, because vintage parts are getting more expensive all the time. A third wheel that I can buy for a modern movement that is $35, for a vintage movement of the same kind is $170 (and that is one that Omega still sells, so it's not discontinued yet).
In the end, you have to do what you are most comfortable with, and if that letting it go until there's a problem., or servicing on a regular schedule even if there aren't any issues, that's your decision.
Cheers, Al
Water intrusion that damages parts like dials, hands, any some movement parts is not typically covered in a standard service fee, so if you get your watch wet regularly, then waiting until it tells you it needs service may not be a smart strategy.