How Long between Sunset and Dark

Some communities end Shabbat 42 minutes after sunset, which is when some rabbis in New York observed that it gets dark. At other latitudes, the interval may vary considerably, from shorter toward the equator or much longer toward the poles.
Note In New York, dark is at 45 minutes after sunset in the winter and 50 minutes in the summer, according to Rav Moshe Feinstein. Some people wait 72 minutes after sunset before doing any melacha.