It's straightforward arithmetic. Look at the milliAmpere-hour rating of the batteries. Multiply by 1.2 Volts to get their energy capacity in milli-Watt hours. Divide by 1000 to see it as Watt-hours. Guessing at 80% charging efficiency, divide by 0.80.
An example of two 1250-mAh AA batteries:
(1250 x 2) / (1000 x 0.80) = 3.125 Watt-hours capacity.
The stove at maximum continuous output delivers 2 Watts. 3.125 Wh/2 W = 1.5625 hours
This is a good ball-park figure, perhaps too optimistic on the charging efficiency and you ability to keep it going at max.
So with a few handfulls of twigs and a couple hours I can ensure my safety lights will have power! I like that idea. Now to find a scale and see how much fuel weighs the same as the stove for comparison.