It's complicated, and can be shorter in some cases, but 70 years after the death of the author is pretty safe. But the interesting aspects come in with, as you say, what publishing means, but also, and interesting to me: what does "composing" mean.

Take House of the Rising Sun as an example that pretty well everyone will have heard. The average person on the street will think it is an Animals song. (I know, I heard their arrangement on a karaoke machine just a week ago!) They got it from Bob D, who got it from Dave van Ronk (who seems to have input the jazzy descending-in-semitones base line), and he got it from... I don't know who, but its roots go a long, long way back.

So there are lots of artists who have *recordings* of HOTRS that are subject to copyright. But if someone sings a version with their own take on the words/melody/accompaniment, they would, I hope, not claim to be the composer. I submit that, as with a lot of musical genres, it simply makes no sense to say that there is or was such a person.