Apple has outlined specific information about Apple Watch battery life on its website



From MacRumors:

Apple claims that the Apple Watch has battery life of up to 3 hours for talk time, 6.5 hours for audio playback over Bluetooth, up to 7 hours during a workout session with the heart rate sensor on and up to 48 hours for timekeeping. Apple Watch charging times are listed as about 1.5 hours from 0% to 80% and 2.5 hours from 0% to 100% using the included MagSafe inductive charger.

  Apple Watch Battery Life: 18 Hours Mixed Usage, Up to 72 Hours on Power Reserve