Just a quick update as I still use this device nearly every day for a few minutes, and by my count it's been running for nearly 1 year and three months off the same battery I put in it when I assembled it! So that actually puts it right in line with my original 1.5 year estimation! Now it is apparent that the battery is getting quite flat at around 2.8V or so and the display is getting dim with the falling voltage so I'm not very confident it'll hold on much longer without a battery swap. Overall I'm pretty happy with the runtime especially considering how often I use the little guy.
*my phone camera makes the display look a little brighter than it does in real life