Apple has outlined that the Apple Watch battery is designed to retain up to 80% of its original capacity at 1000 complete charge cycles, which gives the watch's battery a lifespan of about two-and-a-half to three years based on fully charging the wrist-worn device once per day.
Comparatively, the MacBook and iPad can also receive up to 1000 complete charge cycles while retaining up to 80% of original battery capacity. iPhones receive up to 500 complete charge cycles, and iPods receive up to 400 complete charge cycles before the battery may deplete further.
A teardown of the Apple Watch earlier today revealed a small 205 mAh battery inside the device, which lasts up to 18 hours based on mixed usage and up to 72 hours in Power Reserve mode. The battery is covered under Apple's limited 1-year hardware warranty, while out-of-warranty battery service is also available for $79 plus an applicable $6.95 shipping charge if required for the Sport, Watch and Edition models.