To charge the battery:

Gently set NAO in a crouching position to ensure stability.
Plug the charger to the mains, and plug the charger cable into NAO‘s back. Check the charger’s LED to have an indication about charge status :
When the battery is charged, disconnect the charger cable from NAO‘s back, then disconnect the charger from the mains.

Charging time: 2h for 100%.Battery life: 45min to 2h, depending on use.

How do I know it is time to charge the battery again ?

Notifications warn you that the battery needs to be recharged.

For further details, see: Battery – notifications section in the notifications link above.