[nl]

The decoding is probably in the same order of magnitude as listening to music from your watch.

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?

[kalleboo]

The always-on display drops power usage by being quite dim and reducing the update frequency to 1 Hz. 24-60 Hz video will draw way more power