At full brilliance, the power requirement for those LEDs is around 3W. Three good-quality AA batteries with a capacity of 2500mAh can deliver that for about 3 to 3.5 hours, with gradual reduction of delivery after that point which will visibly reduce the light produced. Ergo, at full brilliance, you can expect a lantern runtime closer to four hours, with the last half-hour at decreasing light levels.
That’s actually pretty useful. These are worth having - but store them with the batteries removed if possible. “Leakproof” is still not a term that can be relied upon with alkaline cells.