Could you also state why you think it should last the time that you mentioned?
Fact is, if you want to notice some serious differences you have to make the exercise last more than 10 or 15 minutes. In real life, that just won't cut it. I would say in general, your workout should last between 45-90 minutes. The lower the reps, the less time for the workout. The higher the reps, the more time for the workout. The reason I state this is because the fewer reps you are doing, the more energy you are putting forth and the more testosterone you are releasing from your body. So it would take a higher amount of repetitions to benefit you and tire you out as much, if possible, to the same degree as a few low repetitions. And the reason I suggest shooting for somewhere in this time frame is because it is just enough to build stronger muscles, it is not overtraining(2 hrs.+), and it is not training too little(30 min. or less), either.