After buying the Nasdaq 100, when should you sell?

This is a private message from a friend who is 30 years old. He’s just getting started with US stocks and doesn’t understand individual stocks, but he wants to do a regular investment in the Nasdaq 100.

After reading it, I felt I had to write something down.

The question sounds very basic, but 90% of people who are already doing regular investing actually haven’t really figured it out.