“As the mmWave sensor enters a low-power state when no motion is detected, it allows the Aqara FP300 to have a 2 year battery life while being powered by a pair of button cell CR2450 batteries.”

Reading through the marketing bullshit since this is the only number mentioned: It’ll last 2 years if there’s nobody ever in that room. In reality, it’ll last a few months at best and probably less than that if there’s people constantly in the room, as the mmWave sensor will need to be powered non-stop.