My Explorist on a stock face with ambient ON will have generally 25-30% left on it, depending on how much I have used it during the day. (notifications, SMS responses, etc.)
When I use any Facer faces, the battery life is affected but it is still a solid day. One day, the battery technology will get to the point that these devices will last multiple days. But hey, we charge our phones every night, so I'm not really concerned about it until it doesn't last an entire day.
I need to turn Ambient off and see how it does.