Hi
The battery in Apple Watch is designed to retain up to 80% of the initial capacity after 1,000 completed charging cycles. Charging cycles work on a cumulative basis. For example, one charging cycle would be completed after:
- Two charges to full, each starting from 50%, or;
- One charge to full from a starting level of 80% plus one charge to full starting from 20%.
Should you require a replacement battery after the expiration of your warranty (and Apple's diagnostic test confirms that the battery's capacity is less than 80% of the original full-charge capacity), Apple offers a chargeable, Out-of-warranty battery service for Apple Watch. You may receive a replacement Apple Watch when ordering battery service.
More information:
Apple - Support - Service Answer Center