To check the battery's level double-click the on-off button. The 5th fret battery level indicator should light solid for 3 seconds, then the lights should turn off. 1 LEDs shown is a low battery level and 5 LEDs is a strong battery level.
The battery will last about 8 hours with all lights on continuously so it will last for a long time under normal use.
It takes about 4 hours to reach full charge from being dead.
The guitar's LED system can still be used while charging.