How long your watch lume glows depends on a few factors. What kind of lume is it? What grade is it? What color is it? How well was it charged up by the light source? How old is the watch? And most importantly, how much luminescent material is on the dial?
That is why this question can’t be answered definitively. It really depends on the watch.
If you are worried about lume not lasting long enough when the lights are off, then consider a watch with tritium tubes. Tritium will provide a subtle but constant glow.
Additional Reading:
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