The results are in.
Both handwarmers were fueled with 5 ml of Poundland lighter fuel (other makes are available).
Unit A was filled last night at approx 2100z and placed in a ziplock bag.
Unit B received its 5ml this morning, 5 minutes prior to lighting it and its partner.
Unit A gave up the ghost after 3 hours.
Unit B expired after 2:55 mins.
The conclusion - fill up the night before and keep in a plastic bag
I expect, in truth, it makes no difference. Obviously, if the unit had not been stored in an airtight enclosure, evaporation would have been an issue, but I wouldn’t recommend storing like so, as everything will reek of lighter fuel.
You can also extrapolate that consumption is about 1ml per 36 minutes, or 1.7ml/hour.
I stuck my temperature probe inside the pouch with the following result.