I was curious too about the battery life and then pleasantly surprised.

"When asked about battery life, reps told me that it should last 36 hours in always-on state, and even longer if you opt to turn the screen off -- there's a companion app that you can download onto your Android device, and it gives you a few settings."

From http://www.engadget.com/2014/06/25/lg-g-watch-hands-on/