I love sunset, that daily disappearance of the Sun below the horizon is so romantic
Yes and it some times incredibly beautiful and full of color.  Have you any idea where all those color come from?
Sun has disappeared below the horizon, until it descends to 6 degrees below the horizon, I think the color red is best to see on that distance  
And at what time on average does sunset occur?
It depends where you live, and what time of year it is. The time of sunset is defined in astronomy.
I know it varies also depending on the time of the year.  Sometimes it happens so late that it seems as is the day is much longer, and you can do much more.
Astronomy
Astronomy (from ) is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry to try and explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates outside Earth's atmosphere. Cosmology is a branch of astronomy. It studies the Universe as a whole
, atmospheric refraction causes sunlight rays to be distorted to such an extent that geometrically the solar disk is already about one diameter below the horizon when a sunset is observed.
Sunset is distinct from twilight, which is divided into three stages, the first being "civil" twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon; the second phase is "nautical" twilight, between 6 and 12 degrees below the horizon; and the third is "