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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.
As others have stated, standard return is 15 days. You do also get the standard 1 year warranty on the Watch and you can get AppleCare if you so desire. If it is a good Best Buy you should not have too many problems returning it if that's what you choose.