Liszt's Grand Solo de Concert typically lasts about 18 minutes. Alkan's Concerto for Piano Solo has a first movement which takes over half an hour to perform (more than the other two movements put together); the Overture from the Op.39 etudes is also roughly fifteen minutes (on Ringeissen's recording I think it falls a few seconds short of 15 minutes). Longer than fifteen minutes are the first movements of the Beethoven Concerti No.3, 4, and 5, of the Brahms No.1 and 2, of the Rachmaninov No.1 and No.3 (on some recordings), of the Chopin E minor, of the Schumann A minor (on some recordings), of the aforementioned Schubert D.960, and probably a number of others. Schumann wrote a lot of suites of smaller pieces which work best when the entire set is played and which last fifteen minutes or more, such as "Carnaval", "Davidsbundlertanze", "Kreisleriana", and "Papillons". His Fantasy in C and Humoresque in B-flat are also around the fifteen-minute mark.
Oh, and the Diabelli variations by Beethoven.
