You can leave a loaded mag on the dashboard of you vehicle, and not have to worry. It takes at least over 400 degrees, for it to go "boom".
Let's put it this way, I would be more worried about polymer guns being in the car (the plastic melting ), than the ammo. But even the the guns can take the heat.
To your question on how long should you wait till you load the firearm. Whatever is most comfortable to you. Me, I slap that magazine in as soon as I get in the vehicle.