The speculation was rampant, and the prevailing rumor was that the Apple Watch wouldn't last a day on a charge. Of course, all of that speculation seemed a bit pointless given that at the time of the announcement, we were probably somewhere around 5 or 6 months away from the Apple Watch release. Today, Tim Cook spoke at the WSJ.D conference, and finally commented on the battery life, saying "We think you're going to end up charging it daily. Overnight — that's what we think."
That's far from a definitive answer, but Cook did explain why battery life was a bit hard to pin down. Cook said that it is a simple matter of the Apple Watch being a completely new product for the company. He said that because "there's no history", it's hard to predict usage patterns and therefore battery life. But, apparently, based on his own experience, you'll charge it every day, "because you're going to use it so much." That's an obvious Applese marketing statement, but it does at least give hope for more than a day of battery with lower use.
source: The Verge