Q: Duration と Period はどう違いますか?
A: Duration means the length of time something lasts: the duration of the flight - the duration of the war - the duration of the parliament.

Period refers to a particular time in history/the past as in the classical period in music or the romantic period in poetry


Q: Duration of the COVID has became too long.

Most people have already lost patience or interest about COVID. この表現は自然ですか?
A: Thank you so much(*´з`)
I didn't know the phrase "far too long" until now.
As of today, I will use it(≧▽≦)
Q: Duration of the course to be a pilot is two years.

In the sentence above, does "to be a pilot" describe "the course" ?
A: Yes! It’s makes sense. Although, maybe change ”be” to ”become” to make it sound a bit more natural! Also, put ”the” in front of ”duration”. So, ”The duration of the course to become a pilot is two years”.
Hope that helps!☺️
Q: Duration of the course to be a pilot is two years. この表現は自然ですか?
A: @TKkorean I would just add a “The” at the very beginning to seem more natural