Two hours, I believe, same as AI.
I really think AI should do the same thing regarding original songs.
I have mixed feelings about this. These days singers often "write" their own songs. In reality, what they do is collaborate with songwriters, and put their name on as co-songwriter because it's an extra revenue stream. The songwriters like this approach because, if they are working with an established singer, they are guaranteed to get their work published and recorded. But few singers are also really first rate solo songwriters.
Do you know what kind of viewership numbers NS has?
I don't, but it is WAY less than AI. NS is on cable, not a broadcast network like AI. Cable rarely gets the ratings of the broadcast networks, and when it does, it's usually only for big breaking news stories. NS is on the USA Network.
The writing part would just be another facet. It could come into play in the event of having two equally talented singers, the ability to write might help that person to squeek by.