Battery life is one of the most important factors of any wearable device. Apple claims that the Apple Watch will deliver around 18 hours of battery power with normal use and after an overnight charge. Early reviews seem to agree with Apple and indicate that the Watch battery will last through a typical day with normal use. This does not take into account using the Watch while working out. Apple says that the Watch lasted up to 6.5 hours when "paired with an iPhone using preproduction software with a workout session active and the heart rate sensor on."
Apple Watch users can stretch their battery life up to 72 hours with the Power Reserve mode. The Power Reserve will automatically kick on when your battery power reaches 10 percent. This allows you to at least use the Watch to tell time until you can get to its charger. You can manually turn on the Power Reserve mode by swiping up on the Watch face and navigating to Power glance > Power Reserve and tapping Proceed.