Sunset itself is an instantaneous event, the moment the upper limb of the sun disappears below the horizon. However, it's more common to refer to the period of twilight, which encompasses the time it takes for the sun to fully set below the horizon and for the sky to darken.

Here's a breakdown:

* Civil twilight: This is the period where the sun is between 6° and 0° below the horizon. It's considered light enough for most outdoor activities. This typically lasts about 30-45 minutes.

* Nautical twilight: This is the period between 6° and 12° below the horizon. The sky is starting to get darker, but the horizon is still discernible. This lasts about 30-45 minutes.

* Astronomical twilight: This is the period where the sun is between 12° and 18° below the horizon. It's the point where the sky is dark enough for most astronomical observations. This lasts about 45-60 minutes.

Therefore, depending on how you define "sunset," it can be instantaneous or take anywhere from 30 minutes to over an hour and a half.

The actual duration of twilight varies slightly depending on your latitude and the time of year.