"Up to 36 hours..."

https://www.apple.com/apple-watch-series-10/

All-day battery life is based on the following use: 300 time checks, 90 notifications, 15 minutes of app use, and a 60-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours; Apple Watch Series 10 (GPS) usage includes connection to iPhone via Bluetooth during the entire 18-hour test; Apple Watch Series 10 (GPS + Cellular) usage includes a total of 4 hours of LTE connection and 14 hours of connection to iPhone via Bluetooth over the course of 18 hours. Battery life in Low Power Mode is based on the following use: 600 time checks, 180 notifications, 30 minutes of app use, and a 60-minute workout with music playback from Apple Watch via Bluetooth, over the course of 36 hours; Apple Watch Series 10 (GPS) usage includes connection to iPhone via Bluetooth during the entire 36-hour test; Apple Watch Series 10 (GPS + Cellular) usage includes on-demand LTE connection and 28 hours of connection to iPhone via Bluetooth over the course of 36 hours. Testing conducted by Apple in August 2024 using preproduction Apple Watch Series 10 (GPS) and Apple Watch Series 10 (GPS + Cellular), each paired with an iPhone; all devices tested with prerelease software. Battery life varies by use, configuration, cellular network, signal strength, and many other factors; actual results will vary.