Anything from minutes to months depending on the complexity of the song and the amount of inspiration you have.
The longest period it has taken to write a book was 10 years. War and peace.
Well some people can just sit down and write a song in an hour or two. Kinda just depends on what mood they are in and if they have some sort of inspiration
He never wrote his first song---his mommy did.
It takes him about 1 and a half years to write a book.