If switched to it's lowest mode, my Nitecore P12 light will go for about 21 DAYS continuously. This low mode is only one lumin though. But you'd be surprised how useful one lumin is if your eyes are dark adapted. But if they aren't, you might not even realize the light is on.

This light has a very wide range of brightness. Modes for this light are:

1 lumin for 520 hours
70 lumins for 28 hours
240 lumins for 6 hours
1000 lumins for 1.25 hours

I have two 18650 batteries for this P12 light, so that would be good for 42 days continuous on low setting. Plus I must have a dozen other AA and AAA lights with a bunch of Eneloops to power them (a couple dozen at least), so disaster illumination is not a major worry of mine, unless the disaster is permanent. Plus, there's always the Costco alkaline battery packs we keep on hand too - we keep AA, AAA, C and D - and as you probably know, you gotta buy large bulk packs at Costco. What are those Costco AA battery packs, something like 60 or 80 AA batteries? A bunch.