I start with about 12-15 lit coals. Too many lit coals will set it up too hot. I would say 10 lbs will handle up to 12-15 hours or so if the temperature stays put. High temps= faster fuel consumption. Once you're cooking, leave the lid ON! Opening the lid allows an inrush of oxygen, which will cause a temperature spike. Too much coal in the basket is better than too little. What doesn't get burned can be used the next time. When you're done cooking, close all of the vents, including the exhaust, and the coals will go out. Next time, knock the loose ash off of the remaining coals, and add enough new fuel for the cook. I've run mine for 13 hours, and still had charcoal left. My basket is 16" in diameter, and 8" high. I've never filled it to the top.
I'm very intelligent, but I mask it by doing stupid stuff.