I have a cover of 'Hallelujah' which, according to Rhythm Box is 870 hours, 34 minutes, 6 seconds. I'm going to go ahead and write that one off as corrupt file info so the next longest is Cassandra Gemini by Mars Volta which is 31:42.

Shortest actual song (sort of) is 'Farewell to John Denver' at 15 seconds from a Monty Python album.