https://www.cnet.com/news/googles-wear-os-update-will-add-battery-saving-options/

The main update to battery saver mode will make your smartwatch display only the time when its charge falls below 10 percent.

Remaining inactive for 30 minutes will make your watch go into a deep sleep mode to conserve power, so you won't have to worry about coming back to an empty battery if you take the watch off.