The manufacturer probably measured the output in the same way Petzl does,

http://www.petzl.com/en/outdoor/headlamps/lighting-performance

i.e. the light output time claims mean very little and are misleading. A lot depends on the type of electronic regulation (if there is any), the battery cell technology used etc.

I haven't yet measured the time this Lantern will take to become depleted, there are to many variables. I don't even know what type of regulation is used as of yet. The packaging doesn't specify any runtime or lumen output specs.

Judging by the light output, I've compared it to 3C Maglite with a 140+ Lumen LED using some LSD NiMh C cells

http://www.thetorchsite.co.uk/TTS_3_watt_LED_upgrade.html

with a specified 3W power consumption, the light outputs seem to be similar when both are bounced of a ceiling.

So assuming constant current/voltage regulation with 4 2000mA capacity Eneloops @ 1.2V this would give 9.6 Whrs. At approx 3 Watts dissapation gives an approximate time of 3.2 Hrs use at 140+ Lumen.

Using Lithium AAs would get you around 6+ hrs use at maximum output.