I Heard...

I heard that a watchmaker that does very similar work to myself, mostly antique pocketwatches, tells people to expect an error as much as +/- 10 minutes per 24 hours. I was surprised, that seems like a lot. It doesn't take extreme effort to get to +/- 1 minute on the majority of watches, and the majority do less than +/- 30 seconds. Occasionally there's a good reason for a larger error, such as with an especially primitive or old watch. But even then +/- 2-3 is achievable in all but the most extreme cases, in my experience, without spending a lot of time. There is diminishing returns for ever greater efforts yielding ever smaller improvements.

More about watch accuracy and rates here...