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All-day battery life is based on 18 hours with the following use: 90 time checks, 90 notifications, 45 minutes of app use, and a 30-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours.
While we don't have any clues to its exact running time, Tim Cook's statements about charging the device at night suggests we could be looking at up to one day of use for the device. We'll have more information about the Apple Watch's battery life as it becomes available.