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.
Yes in time of winter earth is further away from sun. Astronomy  is a natural science that studies those phenomena. 
Astronomy seems very interesting but at the same time somehow over my head