 Depending on the age of the child, minutes. Literally under five minutes, it could kill an infant. Wow. Elvis, does that shock you in that short period of time? It is pretty shocking, but the temperature inside the car is probably about, what, 30, 40 degrees higher than outside? On average, it's 50 degrees higher after 40 minutes. Okay, so you've got all those windows. And it doesn't matter if you crack them.