Wind it until you feel the resistance rise sharply. You won't hurt it by fully winding it, just don't crank on the crown once you reach full wind. If you never reach the point where the crown doesn't want to turn anymore, no matter how many times you wind it, the mainspring is broken. Although many would tell you the watch won't run with a broken mainspring, it depends where the break is - if it's in the right spot (out at the tang end of the spring) the watch can run with a full reserve and keep time quite well also.
Cheers, Al
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