Recording audio needs minimal battery and storage - if you configure it to transcribe into text and transfer the payload every time these things are at the charger, you will never notice any battery drain, only a minimally increased charge time (which you won't notice since they did this from the beginning i assume)

Mic sensing for specific patterns can be super low power. You could set it to speech-like patterns (audio in specific frequency ranges, above a volume limit that makes it audible, with peaks at known frequencies for speech), and turn it on only to record when such patterns are detected.

And since Meta advertises ~9 hours of moderate use (meaning voice assistants, constant phone connectivity, video recording) with gen3, plus 50 extra hours in the charging case... That's easily extrapolated to ~36 hours stand-by audio pattern watching and occasional recording.

They have algorithms that monitor usage. From there it would be easy to set up an interval recording spy capture profile. You might even set triggers on the microphone to initialze the camera if the right conditions are met. In short. Its probably on standby/hybrid-sleep and it records in short intervals.

Let's reframe the question:

How long could our devices last on a single charge if that didn't do stuff like this?