I love sunsets, they are my favorite!
They are very pretty. It is the time when the Sun's disk disappears below the horizon.
Do you know the average time they last?
No i'm not sure how long. Night occurs when the sun reaches 18 degrees below the horizon and you can no longer see it in the sky.
I never knew it was so technical. Do you know why it is so pretty? 
I think it just has to do with the amount of sun in the sky and the way it illuminates.