We're having friends over and I'd like to make 3 or 4 pizzas. I'm considering stretching out the dough and put on the toppings ahead of time, say an hour ahead of when I pop them in the oven. How long can the pizzas sit on the counter in this pre-oven state?
You can stage pan pizzas (chicago style, or pizza hut style) for up to four hours as long as you keep them covered. You can stage a stretched dough for up to an hour and a half between two pieces of wax paper. However, you need to take the time into consideration when proofing your dough so that you don't get blown dough as that will not be delicious. You should not sauce the crusts or top them as pre-saucing will change how your dough cooks in the oven.
If the dough forms a harder crust/ dries out, you should consider tossing it as it will not result in good pizza after cooking.
I wouldn't recommend that; the dough may well overrise and poop out on you, not to mention get soggy from the fillings. Your best bet is to just get the dough rolled out into balls and the fillings all fully prepared in separate containers and the oven preheated. Then, say 10 minutes before you want to fire them, roll out the first one and top it, and do each of the succeeding ones while the previous one is in the oven. Give your guests a drink and an appetizer and they won't mind that you are busy in the kitchen for a few minutes.
We always bake our crusts for 6-8 minutes before we top them. The pre-bake can be done well ahead (and you can even freeze the pre-baked crust) and then topped and baked for 6-10 minutes when you're ready. If you have all the ingredients ready for topping, it can be a very efficient process.
I would store prepared ingredients in separate boxes and 3 or 4 balls of dough in the fridge. Then just before dinner time I would let friends have fun by preparing their own pizza by choosing exactly what they want on.