Battery life varies depending on use and other factors.
In tests, for example, Apple achieved 18 hours of battery life with the following usage: 90 time checks, 90 notifications, 45 minutes of app use and a 30-minute workout with music playback from Apple Watch via Bluetooth. Other examples of battery life under different usage are available here:
http://www.apple.com/watch/battery.html
For the best performance from your watch (including the most power-efficient communication with your iPhone), keep both Wi-Fi and Bluetooth permanently enabled on your iPhone.
If your watch is experiencing shorter battery life than Apple's examples suggest you should expect, try the following steps, which have helped to resolve battery life issues for some users:
If you have not yet done so, update your iPhone and Apple Watch to the latest software:
Close all open apps on your iPhone and then restart both devices:
- On your iPhone: double-press the Home button, then swipe up on each of the app previews to close them.
- Turn both devices off together, then restart your iPhone first.
- Restart your Apple Watch - Apple Support
Unpair and re-pair your watch: