Just to see where you saw 15 minutes I too have the same manual, and it says 15 minutes also.
My Nitro never has come close to run 15 minutes and it seems more like 5 minutes! When I asked a relation that worked at the Dealership where I special ordered mine, he said the time is not adjustable and I think he said something about 7 minutes. Anyway after checking mine out he said it was O.K.?
I will check the running time out on my Nitro and any more info, and report back.
Nitro Year: 2007 (1 of 91,815 sold in 07)
Nitro Model: R/T 4X4 Stone White
CAT-BACK Exhaust, CAI, Projector Head Lamps
Fully-Equipped w/all factory options