If a just serviced watch with a 60 hour movement (23j Ill Bunn Special) is fully wound, then kept stationary in the pendant up vertical position, as the power goes down, normally how long would it be before it starts to lose time?
Thanks, Jack Williams

it should not lose time it will just stop as far as I know it is rr grade watch and was made/design to run same on no matter how much it was wind up,
but I might be wrong

That would be on a watch to watch basis and depending on whether the mainspring is new or used and how well,it is adjusted, different positions, along with a thing called "isochronism".

Isochronism is an adjustment that makes the watch's rate immune to amplitude of the balance wheel's swing. When a mainspring is fully wound, the balance wheel swings wider and generally around 270 degrees end-to-end. As the tension of the mainspring runs down, the amplitude decreases, until it swings shorter distance end to end and generally around 105 degrees. Without being ridiculously technical and at the end of my rope, the Breguet overcoil on the hairspring, serves almost the same purpose as a linear oscillator, whose frequency is independent of its amplitude, at least in theory, in manual wind watches, and is usually fairly predictable as long as the watch is wound at the same time every day.

regards,
bb

After 48-50 hours you will see some small changes in the time keeping. The 60 hour design Illinois came up with was a great improvement,, up till then most all watches were 40-48 hour watches,,, if you forgot to wind your watch in the morning the next morning it would be either running with very low amplitude or stopped running at all...On the 60 hour watch it would still be running if you forgot to wind it. and have several hours running time left before the watch would quit.

The 992b was a 60 hour watch even though not advertized as a 60 hour watch..The mainspring in a 992b is longer than in a 60 hour illinois watch.

Peter, Buster, Samie - Thanks to all of you for your answers and taking time to share your knowledge with amateurs like myself - it was most interesting!