Lifetime, I mean battery life.
Battery life can still mean two things;
(a, runtime) How long will it last on a single charge, and
(b, durability) how long until it's wore down, and needs to be replaced with a new one.
A, runtime: Really depends on how much of a load you put on it. Typical runtimes are often between 45 minutes and 250 hours or so, but there's a lot of variation. Also, 2 hours of runtime might mean it'll last someone about a month or three for example, if it's not used too frequently. A lot of lights these days have different modes, draining the batteries at different speeds. That also means the same light could last between 1 hour and 100 hours, depending on how you use it.
B, durability: Somewhere between 6 and 36 months perhaps. I'll typically buy 2-3 batteries with anything new (if I don't have enough batteries for it from before), and then buy another 2-3 batteries a few months after that, just to have replacements ready at hand. There's a huge variation here, some will ride their batteries really hard on a daily basis, others will just have a low load on the once in a while, and so on. I budget for replacing them every 6 months – a bit pessimistic – with the intention of having a small stock of them, and then slowing down purchasing, to not let new batteries age too long on a shelf.
And please do note that I'm just throwing out some numbers to give you a bit of a ballpark impression here. These numbers are not intended as "the" truth, and I'm sure we could run into a heated debate about these things. You won't have to look far to fine something outside of those numbers, or someone that'll disagree with them.
I'm trying to draw up a picture of ballparks here, nothing more.