One candidate is Ray Stevens' 'Jeremiah Peabody's Poly Unsaturated Quick Dissolving Fast Acting Pleasant Tasting Green and Purple Pills'. Talk about typist's cramp!
Don't know if its the longest but it's a good one: 'Several species of small furry animals gathered together in a cave and grooving with a Pict.' Frank Zappa
If you're looking for UK chart entries, how about Meat Loaf's '94 hit 'Objects in the rear view mirror may appear closer than they are'? Or, possibly bending the rules slightly, 'Where the streets have no name - can't my eyes off you / how can you expect to be taken seriously' by the Pet Shop Boys in '91?
Sorry, Graeme, but 'Several species of small furry animals gathered together in a cave and grooving with a Pict' is by Pink Floyd. On Ummagumma. Not Frank Zappa. Amusing though -- and it has a Hendrix reference in it. Anybody know what it was?
I believe that the longest song title ever is: "How could you believe me when I told you that I loved you when you know I've been a liar all my life?".