Apple has been quiet on its Watch's battery life, but it doesn't sound like you'll want to go more than a day off the charger. "We anticipate that people will charge nightly which is why we designed an innovative charging solution that combines our MagSafe technology and inductive charging," Apple spokesperson Nat Kerris told Recode, which quotes a separate source as saying that the Watch lasts "about a day right now."
The Apple Watch won't see release until early next year, so there's still some time for Cupertino to tweak its power efficiency. And if it can reliably make it through a single day, that would still be an improvement on Motorola's Moto 360 — though other Android Wear watches like the Samsung Gear Live and LG G Watch have shown slightly better performance. Either way, you shouldn't expect any sort of breakthrough in this regard from the Apple Watch.