I like to not have to worry about things. Particularly the batteries in a rifle or pistol I depend on for self defense. If you cannot count on it, you should not carry it or use it. It is basically a paper weight. The one thing I hear a lot is when people say "oh yea this optic lasts xyz many hours!". While it is nice, I will never push the limit of how long an optic illumination lasts. It is not worth it! So in regard to my red dots or optics with illuminated reticles, I replace those batteries before the said amount of time they last. An Aimpoint T1/T2 for instance stays on forever, literally the best red dot optic in my opinion. I change it out once a year and I leave it on all year. I never turn it off, because when I grab my rifle I do not want to fumble around trying to turn it on and to whatever setting I want. This is why I recommend Aimpoint red dots. I have a Primary Arms and it might last 6 months at most. I cannot depend on it like I do my Aimpoint's.
So what I do it make a quick list of my optics and flashlights, etc. that require batteries and how long they last. Then I always replace them halfway through their life. At least according to what I have seen in use. A battery is a cheap, buy tons of them and replace all things needing batteries on your rifle/pistols. Do not let a battery be the reason you get dead while trying to protect yourself!