The stones are extremely thick and expensive. I had it in there for about an hour. The grill was just under 500 and the stone never got above 375 according to my thermo laser gun thingy. (that's the technical name for those, right?)
Hogs, if the baking stone never got above 375, I would heat it longer before cooking. I will heat soak the KK, pizza stone in place, for at least an hour. There is a lot of mass there to absorb heat. If you heat soak effectively you don't lose much heat if you are cooking multiple pies. Plenty of approaches, this is what works for me.