Mikle Oldfield's Tubular Bells (if you count it as a "song") wiht two parts eaxh in excess of 20 minutes is a contender. Amazing to think this was a best-selling album, can you imagine that these days?

Someone mentioned the shortest song as being 16 seconds, evidently they have never heard of Napalm Death, whose "You Suffer", consisting of the words "you suffer" grunted very quickly along with a stabbed guitar chord clocked in at a miniscule 0.5 seconds. Granted, it wasn't a big hit.