'Manmohini morey' from Yuvvraj is the longest Rahman song that I know of. The last time I heard it, it ran a continuous 35 minutes or so. Impressive for a song that Rahman originally tuned to run about 3 minutes. The song is a techno marvel sans the meaningless repetitive lyrics that always seem to accompany such beats. What lyrics there are, are sung in a classical style that one expects will pair with those beats like Wall Street and common sense. But the combination is an aural revelation which I would liken to to a beautiful fractal equation, if not for my utter mathematical ineptitude. This song might beat Yakkai Thiri for the longest Rahman song in my library. However there is no way it will get close to the sheer number of plays which stands closer to 10s of thousands.
In case it interests you, I also mused about the really short pieces earlier in this piece