Once the sun touches the horizon, it takes approximately 2 to 3 minutes to go below the horizon. This is dependent on several factors, including latitude, time of year and atmospheric thickness.
As the earth rotates, the sun appears to travel across the sky at a rate of 1 degree per 5.6 minutes. The disc of the sun measures approximately half a degree in diameter. Latitude, however, plays a big part in this; the closer to the equator, the more directly and quickly the sun sets. The thickness of the atmosphere that the sun's light has to travel at the horizon also alters the amount of time it takes for the sun to set. At latitudes further from the equator, sunlight has more atmosphere to travel through and thus appears to move more slowly.