I doubt there is any standard answer, it probably depends on how quickly your stone gives up heat to the pie, how quickly it can regain the lost heat in general, and how much heat your oven can put out. If you are baking for home use, you clearly would not want to wait 45 minutes between pies, unless you eat very slowly, and I would load another one as soon as the second pie was made, which is usually only a few minutes after the first one came off.