It lasts longer than you need it to....after about 12 hours or so it seems the requirement is to sweep away a bit of the ash at the bottom but not always. When I plan to smoke for 10 hours or so I actually block off a portion of the grate with a Weber basket ( learned that one from Brett ) so I don't use that much charcoal as I don't need it. I haven't gone over 12 hours yet so I really don't know. I did see a video online when this version just came out and I think they jammed it full and kept it low and if I remember right it was over 40 hours or something silly like that....
At 40 hours it wasn't really at cooking temps anymore but still producing some heat.
A bunch of us have this smoker now and if I am wrong anywhere I am sure I will be corrected....
The first time I smoked I filled it right up and had around half left when finished.....I wouldn't make it a concern of yours....the best way to learn is to fill the bottom up, follow the instructions laid out a few months back in this thread about how much to light and keeping low temp...ect.....and watch how long you cook for and then how much charcoal is left over......there really is no waste as most of the leftovers can be reused for next cook. You will be set for all future cooks after that one.....this one......this weekend.........