Smartwatches have had full HD h.264 and 720p h.265 hardware decode since at least 2014.
https://www.slideshare.net/jjwu6266/qualcomm-snapdragon-400b...
The screen brightness is the limiting factor. Based on always-on displays on the Apple Watch you are probably looking at 8 hours or so?
So decoding with the screen on might be 5 hours?