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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.
Apple designed Apple Watch with a battery life expectancy of 18 hours per day, and from my own experience, I can definitely attest that it usually far exceeds that number. But of course, it all depends on your usage. Depending on what you do with it, you might see better or worse battery life.