Same here, mine have lasted well beyond a year, no sure what brand of battery I have in there. Interesting, on Tuesday, I had checked 2 of my AirTags and they were showing a battery level, while on 2.0.61 and in fact, showing red, then later in the evening, I though I might replace them, suddenly I see they were updated to 2.0.73, but now, no longer showing any visual battery level at all. Here today, gone today! LOL!Mine are lasting well over a year. Probably closer to 2.
Not sure why pretty much every other Apple device you can clearly confirm the current battery level, but yet this device they seem to think despite it's importance, it's not necessary. I guess when you are in the middle of a great globe trotting adventure, they'd rather the battery die on you, or finally get notification of it needing to be replacement, but now you are somewhere that such batteries might not be easily available.
I'll have to assume, it's like most Japanese cars, when the tank show empty, even the digital gas gauge showing 0mi/km left, it does in fact have a surprising amount of reserve.