Found it...
This is the formula passed to me by Albert Einstein's ghost for figuring out how long a bowl of tobacco should last:
Begin by determining the volume of your chamber...
V=πr2h
Then continue by determining the moisture content of your tobacco...
MC=[(w-d)/d]x100
After that, calculate the heat release rate (HRR), that being the burning surface area plus the stacked area (volume of chamber)
Q=q0(Am+As)
Fortunately, ignition source variable is not a large concern.
Finally, calculate the air flow between the two cylinders.
A1 x v1 = A2 x v2
This is factoring an average breathing rate of 18 breaths per minute.
This formula does not however take into account meteorlogic factors such as ambient air temperature, dewpoint or windspeed.
YMMV