My favourite head torch is my Thrunite TH10, built like a tank, runs on 18650 and in my last longevity test at moonlight mode it was on for 2 months and battery was only down from 4.2v to 3.6v, 3.6v being considered an 18650's 'resting' voltage. Right now I'm doing the test again till it turns off :-) will post back in a few month when it does.