There are a bunch of weird smartwatches that get pretty close to this - for example, the Amazfit Bip S Lite claims 30 days of battery with a similar transflective screen (albeit I think the heart rate monitor drops to once every measuring 5 minutes in battery saver mode).

Similarly, the Withings Scanwatch claims the same, although as a hybrid smartwatch, it is not driving such a power-hungry screen.

I had the OG Bip and got extremely close to 3 months at one point. Unfortunately I don't remember if I had the HR set to 5 minute intervals but if it was on, I think that was the setting.

Even with 1 minute intervals though, battery life was absolutely stellar. I'd often lose my charger, and in few days after I saw the battery dropping/low battery alerts, I'd find it too (before the watch died)!

I would expect an extremely infrequent heart rate sample rate at idle. I believe the current-gen watches only sample every 10 minutes unless activity is detected.

it'll probably pulse on / off. The main draw would be the display but since it's e-ink it only draws when changing.

I guess so. I've had two previous fitbits and I believe those track every second during exercise and every five minutes otherwise. But those watches would still only get 5-7 days of battery life.

Hence my skepticism that this feature can be offered and still get 4x more life than that.

it's e-paper (reflective lcd), so it does draw some power at all times, but lower without an always on backlight

I don't know if it's the same thing, but Sharp Memory LCD has low quiescent consumption and memory somehow built into the display.

https://sharpdevices.com/memory-lcd/