Music can be of any length, for example, John Cage wrote For As Long as Possible, which has few notes, sustained for an indefinite length, but it makes sense to narrow it down to recorded music. Classical pieces can go for hours and some have lyrics, such as church music, but also secular music and then their are Operas (is that one long song or several different songs, as they segue into each other, but I guess all CDs do that to an extent), but I suppose we're talking about popular music. Jazz compositions and even some live recorded rock songs can go on for 30 minutes and even in pop, Cat Stevens had Foreigner Suite, which goes on for 18 mins, but I don't think that's really one song. As the post says about one song, it may be best to think in terms of singles and I think the longest of which was Jesus of Suburbia by Green Day at 9mins, so it does raise the question of why 5 was chosen, but we get the idea.

To answer that in the main post about different versions of the same song, are rip-offs and knock-offs included as the same song or not? I think the only way is to listen to every version of Summertime and analyse the digital code. The furthest apart may define the boundaries, but that means at one point a different song is only going to the same distance away as another version of the same song, if it exists yet and we don't know yet what noticeable difference one bit makes in terms of pitch, tempo, rhythm, etc.