Jim - That's a good question.
I have run it with several inches less charcoal than this, and gotten about the same run-time. So, I am wondering if the run time is more a function of time than the amount of charcoal in the reactor.
Clearly, having a "full tank" would tend to slow the diffusion of heat somewhat. But if a metal drum is 500 degrees at the bottom and 90 degrees at the top, I am thinking it is just a matter of time before the whole thing is hot.