Considering they use CR2032 batteries, I’d venture to say that they should last about a year. I wouldn’t necessarily trust the reported battery levels as ST has been making a lot of changes to calculating battery percentages in the last few updates.
My two presence sensors have been on the same batteries for about 6 months now and they are at 63% and 75%.
Please, do tell (in great detail) what you have done there! I am so sick and tired of replacing fob batteries.
I used two AA batteries, lithium because they work better in temperature extremes, the battery holder is just a few bucks, and I soldered the positive/negative wires onto the corresponding battery connections, you could even just use tape to hold the wires down, it’s such an easy mod even a girl could do it probably, it’s been about 10 months now and still showing 100%
The only problem with that is my FOBs aren’t in the car. They are on my and my wife’s keychains (we do a lot of walking). So, were I to get ones for our cars, I would absolutely do this mod. I’ve thought about coming up with a custom enclosure that would fit on a keychain but also allow for a larger battery (maybe a CR2450) or even doubling the CR2032, but I don’t have access to a 3D printer (yet).
A year after, but still relevant IMO, I replaced the battery in one of mine, 2 days later it was 90% 2 days after that it was 70%, 10 hours later 50%. As far as logging goes, those values are the only battery entries reported. In 5 days, there’s 43 events recorded including the battery updates. Nothing like your 20 seconds.
I believe the two I’ve got in active use are the “latest” versions as of this post (v2?). I have one of the older ones that I keep around (though the battery is quite dead) for a ‘guest’ key if someone were to house sit which lasted quite a bit longer - though it sat in a drawer so I’m not sure if there’s an accurate correlation to be made there.
@Alwas I have one I keep in my backpack with a 18650 battery - though its paired with a hub I have at work (too bad the two hubs can’t share devices). I don’t recall exactly when I strapped them together, but it was at least 6 months ago… Still 100% 
Not that surprising I guess considering the ridiculous capacity difference. Would be nice to be able to stuff a CR2450 in one though (@Core_Phx) … I don’t know the specifics, but given the visual size difference and the same voltage, it seems like a logical conclusion that the capacity is larger and would therefore (without knowing more details) last longer.
Trouble is keeping it small and/or light enough to make it worth keeping on a keychain. I have a hard enough time explaining to my GF what the purpose is - I think she thinks I’m using it to track her… really I just want to make sure the lights SHE leaves on when leaving, get turned off… lol
No good answers… maybe a new version? Eh Smartthings?? hint hint? 
I found an even better option for people who want to leave it in a car or a Motorcycle. Someone made an emulator that you can hook a micro USB cable to and it comes with a CR2016 and CR2032 adaptor that sits on top of the CR2016. Check this link out.
