What is the duration of the red twilight and the white twilight from the beginning of sunset, and is this duration fixed or does it change throughout the year?

The consensus of jurists indicates that the twilight (shafaq) that determines the beginning of the 'Isha prayer time is the red twilight, which disappears first. When the redness vanishes, the 'Isha time begins, unless the horizon is obstructed by walls or mountains. In such cases, one waits until the white twilight disappears, as this indicates the disappearance of the red twilight.

The duration of the red twilight varies depending on the length of the night, and the white twilight may persist until one-third or even half of the night.