George Gershwin - Rhapsody in Blue
Mozart - Requiem Masse
Ralph Vaughan Williams - Fantasia on a Theme
Franz List - Faust
Richard Wanger - Tristan and Isolde (Prelude and Liebestod)
Stravinsky - The Rite of Spring
Charlie Parker - What is this thing called Love
(Actually there's a ton of Jazz music that goes well past 15 minutes, Charlie Parker, Chet Baker, Gerry Mulligan, Miles Davis, Art Tatum, John Coltrane)
Several full length (60-90 minutes) mixes by Timecode (Moving Shadows), LTJ Bukem, DJ Shadow & Cut Chemist (Product Placement, Brainfreeze sessions).
Led Zepplin and Pink Floyd both have seriously long versions of some songs. Atom Heart Mother, Astronomy Domine, Echoes, Whole lotta Love, Dazed and Confused, to name a few.