Ask how often a mechanical watch needs servicing and you will get a confident answer, and the next person will give you a different confident answer. Both will sound authoritative. The spread between them is what the answer is.
Why the range is so wide
Manufacturers’ own recommended intervals run from about three years to about ten. That is not one industry disagreeing with itself for no reason, and it is not evasion. Several genuine variables sit underneath it.
The oils are doing real work in very small quantities, and how long they last depends on the formulation, on how warm the watch is kept, on how much it runs, and on how well the case keeps dust out. A watch worn daily runs constantly and keeps its lubricants moving. A watch worn twice a year spends most of its life stopped, which is its own kind of unkind.
Then there is what a service is for. Some of it is lubrication, some is cleaning, some is replacing parts that wear, and some is checking the seals. Those do not all come due on the same schedule, and a maker quoting a longer interval is often making different assumptions about which of them you will attend to separately.
So the honest answer is a range, and the useful version of the question is not “how often” but “what is this particular watch telling me”.
What a rate can tell you, and what it cannot
Here is where measurement earns its place, and where it needs to stop.
As lubricants age, friction inside the movement changes. Sometimes that shows up in the rate. A watch may drift away from where it used to sit, or become erratic, swinging further between readings than it used to. Either is worth noticing.
But plenty of watches do neither. A movement can be well overdue for attention and still keep excellent time, because a rate is the sum of several effects and they can cancel. Equally, a watch can move for reasons that have nothing to do with lubrication. A magnetised watch or one that has taken a knock will shift sharply. A watch that simply spent the measured interval running down in a drawer will shift gently, and neither is telling you about the oils.
So a rate is not a service indicator, and nothing here or in the app claims otherwise. Being outside a band does not mean a watch needs work, and being inside one does not mean it does not.
What a rate is genuinely good at is noticing that something changed. A watch that has sat at +4 for two years and now reads +19, measured the same way in the same conditions, has told you something happened. It has not told you what.
Take a reading before it goes in
This is the small piece of advice worth acting on, and it costs nothing.
Before a watch goes for service, measure it. Two readings a couple of days apart, in a known resting position, is enough. Write down what you got.
You will want that figure for two reasons. It tells the watchmaker where the watch actually was, in your conditions rather than theirs. And it gives you something to compare against when the watch comes back, which is otherwise impossible to do honestly from memory.
What happens afterwards
A freshly serviced watch is usually adjusted to a rate the watchmaker is happy with, measured on the bench. Expect two things.
The first is that your lived rate will not match the bench figure exactly, and that this is normal rather than a complaint. The bench measures a few seconds in one position with the watch undisturbed; your wrist measures days in every position with the watch living a life. Timegrapher readings and real world rate goes through why the two legitimately differ.
The second is that the number may move a little over the first weeks as the movement runs in and its wear and winding become regular again. Accounts of why differ; that it happens is ordinary. A watch that reads slightly differently a month after a service than it did in the first week has usually done nothing alarming.
What is worth watching is where it lands and whether it stays there. A rate that settles and holds is the outcome you paid for. A rate that keeps moving after a couple of months is worth raising, and most watchmakers would rather hear about it inside a warranty period than after.
The other reason to keep measuring
Once a watch has been serviced and has settled, you have something you did not have before, which is a trustworthy baseline in your own conditions.
Everything afterwards is measured against that. It is the version of “does my watch need a service” that a rate can actually contribute to, not as a verdict but as the first sign that a conversation might be worth having.
What is good accuracy for a mechanical watch covers what to make of the number itself once you have one you trust.