In soliciting questions for our Apple Watch FAQ, many people asked about battery life. Apple, no doubt aware of the concern, has published a pretty thorough evaluation of what you can expect from the Apple Watch in a variety of different use cases, including general use, working out, using it as a watch, and so on.
The general conclusion? Around 18 hours of life with a combination of time checks, notifications, app use, and workout with music via Bluetooth.
Of particular note:
Apple Watch battery performance claims are based on test results from the 38mm Apple Watch. A 42mm Apple Watch typically experiences longer battery life.
Not enough, it would seem, to be worth measuring, but probably a small increase. A larger case, after all, means a larger battery, but also a larger screen to power.