It takes Ping a few hours to charge its battery that will last for anywhere from 3 weeks to months depending on your settings. You'll receive a notification when the battery gets low.
Ping was designed from the ground up to maximize the battery life. This is one of the core reasons why we are a locator and not a tracker, which requires constantly checking in every X minutes to report its location (a process that quickly drains the battery).
Ping is constantly in a "low power standby" state, which keeps battery use low while maintaining the ability to receive a location request. When a location request is received, we selectively turn different parts on to obtain the device's coordinates and transmit them back to you. (We actually are patent-pending on this!)
So one of the key factors to battery life is use. The more you locate Ping, the faster the batter drains. We do a lot to try to optimize this, but that's just simple math with any electronic device - more use, less battery.
The other factor is cell coverage. If you're in an area with poor cell coverage, it could further deplete the battery, but the standby mode helps.
We have a number of other "features" in our app that help with the battery life as well designed to conserve battery when the device is not in use (i.e. when you are sleeping) and if you are only using it as a safety device. In those instances, you could get many many months of use.