The exact duration of a sunset is difficult to determine exactly; the exact time will vary by geographical location and season. The very concept of a sunset is itself difficult to pin down. Theoretically speaking, the sun is in the process of setting immediately after it reaches its highest point in the sky.
The technical term for the time immediately preceding sunset is twilight, and this is the time most associated with bright, colorful skies. Twilight itself is broken down into three categories that, according to California Institute of Technology, have been arbitrarily determined. These are civil, nautical and astronomical twilight, with civil twilight providing the most light and astronomical providing the least. Stars are not yet visible during civil twilight.
Both season and location impact sunset duration, meaning that even the same place on the planet will have different lengths of sunset throughout the year. The only place this isn't true is at the Equator, where twilight lasts for a couple of hours every night all year round. By contrast, the Arctic circle experiences the most dramatic variations in twilight duration on the planet, going from no twilight during the all-day sunshine of summertime to as much as four hours of twilight during the dark winter months.