Even though he did say "under" 15 minutes, I still assume he wants a fairly substantial piece. I wouldn't recommend ending a program with La Campanella or Grand Galop Chromatique, although both are acceptable encores if such a thing seems appropriate. If a piece like the GGC were to be programmed in a recital, specifically, I probably wouldn't put it near the end, but rather as an opening to the second half, say, or maybe as a piece before intermission. For the ending piece, however, I think something between 8 and, well as he says, 15 minutes, is the best way to go. But that's just my say...