Friend and meteorologist Jan Null has researched the question and learned:
I discovered that in the first 10 minutes the inside temperature rises an average of 19 degrees, and it doesn’t matter whether the outside temperature is 72 or 90, the heat curve is about the same; that surprised me. In the next ten minutes, it rise another 10 degrees. So in the first 20 minutes, you are 29 degrees above the outside air temperature. And it starts climbing as soon as you shut the door, even if the AC had been blasting.
If the temperature is 100° outside it is 129° in the car in just 20 minutes!