First of all, how much light will you need? Do this by describing how you plan to use your flashlight and under what conditions, and/or by comparing to known references.
Yeah we need more info also of even very small lights will last along time on a 2lm mode (days) but if you need a bright light you will have to get something that takes a lot of batteries or if its a mulit-day event you might wanna bring extra batteries.
The longest run time I have seen is a Quark AAx2 this test revealed 1272 hours on moonlight mode [with energizer ultimate lithium]!!!!! use it a hour a day and it technically should last three and one half YEARS on one set of batteries!!! Shut up!!!:huh:
PAlight Star or Survival models on the "locator glow" setting (a low but usable moonlight setting). With the supplied cheapo battery it lasted about 18 months (as the name suggests, constantly on), so possibly as much as 5-6 years on a decent lithium primary battery.