15 minutes is probably a rough estimate. I'm sure your environmental conditions, e.g. temperature, humidity, etc ..., greatly effect the cure time.
It took me approx 15 minutes to apply NXT to my full sized truck by hand. Then I removed it from half of the truck, waited a few minutes, then removed it from the other half with very good results!
I concur, I used it this past weekend and did a small test on my own. I applied to the whole car, waited 15 min, then 30, 45 and finally 60. 45 and 60 were no different but by the 30 min mark it was signifigantly easier to remove than at the 15 mark.