How accurate should a vintage watch be keeping time?
One question I am consistantly asked when selling vintage watches is 'Does it keep good time?. This question is a difficult one to asnwer without knwing that person's definition of 'good time'. If you were to buy a new chronometer rated wristwatch, this should 100% keep time to well within the COSC specifications. If however your watch is not a chronometer there does not appear to be a strict +or - tolerance for which to measure against, therefore whether a watch keeps good or bad time is often dependant on the owners expectations and lifestyle.
If you pay a high some of money in order to wear a luxury vintage wristwatch you should expect it to tell the time accurately. If however you expect it to keep time as well as a quartz watch or your mobile phone, regardless of how prestigious or expensive the watch, you will be dissapointed. Watches from the 50s, 60s and 70s were built with much more human involvement and much less super high tech machinery, so could not go through near as many QC measures as modern mechanical watches do. Also it's likely if your buying a vintage watch it's down to design, heritage, history, vintage charm and possibly nostalgia, probably not fr accuracye timekeeping.
So at what point does a watch keep 'bad time'?
This is a partially subective statement but realistically it's more likely to be a pain to the average person once it is out by a couple of minutes. If once or twice a week you have to take 2 seconds to move the minute hand back or forward a minute then to me this is really no disruption at all to my week. Therefore I would say that a well made mechanical watch vintage or not should not be losing more 30 seconds per day (3.5 minutes a week). If it's more than this it needs regulating. This is assuming you are wearing it everyday e.g. for work. If you do not you'll probably need to reset the time anyway if the power has run out.
One sensible thing to do with any everyday mechanical watch is simply to check it against your phone at the start of the week and you shouldn't need to touch it again.
Some collectors have enough watches that they only wear each one maybe once a fortnight so unless it losing 3 or 4 minutes a day they won't notice anyway!
The first thing to think about it is when and how you plan on using your watch. If you work in Air Traffic Control at Heathrow airport it probably wouldn't be wise to wear a 60s Rolex, even if you do have the best looking wrist in the office. If your life is dependant on being on time then it's likely any watch more than a few seconds out a day will irritate you and you'll consider it a bad timekeeper. If however you collect watches and only wear them evenings and weekends then a minute a day out probably wouldn't bother you a lot.