It should approximate the amount of time of twilight, or maybe double that, starting when the sun starts to go below the horizon. (Seems like there ought to be a technical word for that, but I can't find it.)
End of Evening Nautical Twilight (EENT for short). Well, aviators call it EENT, there are actually three types of twilight: Civil, Nautical, and Astronomical.
You can get more information from the Navy on the different twilights.
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