How long is it safe to keep dry cat food in a hot car?

I need to get more dry cat food at the store, it would be more
convenient to pick it up first rather than last. If I pick it up first,
it will have to sit in the hot car for at least 2 hrs. I wouldn't think
it would be any longer than 3 hrs though. I don't know how hot the car
would be but I know it will be around 90 degrees Fahrenheit outside and
so it would be hotter in the car. The food will be in one of those
plastic Hill's Prescription Diet bags.

I really don't think it would be a good idea to leave it in the car so
long, especially since it's a prescription food. But I thought maybe
someone would know for sure.