The Apple Watch Battery:
Apple has announced that Apple Watch Battery should last for 1000 charges while maintaining up to 80% of its capacity.
Apple Watch Owners, Your battery is designed to retain up to 80% of its original capacity at 1000 complete charge cycles. The one-year warranty (for Apple Watch and Apple Watch Sport) and two-year warranty (for Apple Watch Edition) include service coverage for a defective battery. If it is out of warranty, Apple offers a battery service. Prices and terms may vary.
If your battery is at its end of life and you are not under warranty, Apple will replace the battery for $79, no matter which model Apple Watch you have.