I find the much simpler "3 up, 3 down" rule is a good starting point for charcoal distribution. What that means is you use 3 coals more than the diameter of the oven on top, and three fewer on bottom. So, if using a 12-inch oven, you would have 15 on top (12 plus 3) and 9 on bottom (12 minus 3). This method will get you to roughly 325-350°F, depending on weather conditions, etc.
A charcoal briquette will last about an hour, depending on the brand you use. If I'm cooking something that takes longer than an hour, then I start some fresh coals about 15 minutes before I need it (I leave a few briquettes burning in my charcoal chimney just for this purpose). One mistake many people make is to overstart their coals, and they're half burned away before they start using them.