It can occur in a very short period of time, and the time shortens exponentially as the temperature gets higher. It can occur in as little as a few minutes, even with the windows open. If you suspect that this has occurred in your cat, I would suggest getting Tippy evaluated by a veterinarian immediately because serious organ damage can occur in heat stroke cases. Your veterinarian will need to stabilize Tippy, and then may recommend diagnostic testing to evaluate how her body systems are working (such as blood work). She may require hospitalization depending on the severity as well.
We do not recommend leaving any animal unattended in a car in warm weather, even if it doesn't seem "that hot" outside, or even if it is just for a few minutes.