i don't think there's a limit to how long a song is. the file size maybe, but i doubt it's too long or something.
Yes, I don't think there is a limit to the length of a song.
The longest song I've got on a map is 40 minutes and 10 seconds, it's not far from an hour. An hour is a bit long to be waiting for a whole song to play out in a map. I would suggest leaving your character in the map, minimise Feralheart and do something else on the computer and you should be able to listen to the song with the program minimised.