S.O.D. put out a song called "The Ballad Of Jimi Hendrix" (which started with the opening riff of "Purple Haze" followed by the vocalist, Billy Milano shouting "YOU'RE DEAD!" (4 seconds) From their low-fi 1985 politically incorrect thrash metal classic Speak English Or Die.
But the track that follows on the album, "Diamonds And Rust," is even shorter.
The lyrics: "Diamonds... And rust!" (Fin)
Now, in terms of modern radio airplay, the shortest songs I have heard often have been The Foo Fighters' "Big Me," which is 2:08, IINM. Del Amitri's "Roll To Me," an AC stalwart, also clocks in at about two minutes.