Jeeze, talk about a question with a lot of variables...
There are so many things that affect run time that it's gonna be hard to get an kind of real ansewr. Motor, ESC, battery make-up, quality, voltage and mah capacity, wire size, gearing, tires, driving surface and your driving habits will all make big differences in run times.
Not knowing any of those things for your car I'd say that anywhere between 5 minutes and 35 minutes could be normal for you.
If everything is stock and you don't go full throttle the whole time and you have a decent battery then I'd expect 15-20 minutes to be about right. But you do notice all the "if's" there, right?