According to Apple, 72 hours.
I was quite looking forward to it, but from what i've read, it disables wrist motion to turn on the screen, and requires a full restart to get out of Energy Saving mode.
Think I'll just use airplane mode when battery is tight.
Apple states that Power Reserve mode is enabled at 10% and from there can last for ~72 hours. I've verified this claim by letting my watch get to 10% battery and then checking the power reserve Usage stats in the Apple Watch app on the iPhone (Apple Watch App>General>Usage>Power Reserve) and even at 10% battery it said the watch would get >2 days of battery in power reserve mode.
Doing a bit of math would lead me to believe that if you can get 72 hours of battery at 10%, you can get 720 hours (30 days) of battery in power reserve mode from 100% battery. That's pretty damn awesome if you ask me.
Absolutely correct. Btw - I did let my battery drain down today - or better it did drain down to 3% at which power reserve was activated - not at 10%.
In any case - 30 days is amazing. Because it means you can easily use the watch just a watch if required. No second watch needed.
I am thinking here mainly for travel situations in which I cannot charge my watch daily.
If you force it into Reserve Mode (hold down side button, slide Power Reserve option), can you get it out of Reserve Mode without having to power-cycle it completely? I know it's 75 seconds or so, but it seems like a waste of battery power to do that.
My watch is currently at 4% (wich is the smallest percentage for the Apple Watch to be in the normal mode) and the App still shows >2 Days for Power Reserve. If this is true the watch can actually be used for 50+ days in Power Reserve mode.