Tim Cook, when asked by Brian X. Chen why he skipped over the watch’s battery life:

I don’t think we skipped over it. I addressed it in the presentation myself. We think that based on our experience of wearing these that the usage of them will be really significant throughout the day. So we think you’ll want to charge them every night, similar to what a lot of people do with their phone.

Looks like battery life will be about a day. Not too surprising given that they didn’t mention battery life on stage and its right around what I’ve seen others guessing before Brian X. Chen’s piece was published. I understand that Apple is fighting against physics with this thing — you can’t put a bigger battery in a device that small. But, the Apple Watch has to be magnificently useful if they expect people to get in the habit of charging a second device every night.