Gruber's review has it anywhere from 30-50% with moderate use at the end of the day. Only time it got below 10% was when he was doing "heavy testing". http://daringfireball.net/2015/04/the_apple_watch
At 3:10pm, this reviewer's update says, "It’s well after lunch. I’ve had this thing on my wrist for something like six hours now, and the truth is that I’ve barely used it."