The lamp should be charged for at least 8 hours before first use. After that, 3 hours of charging will allow the Edison the Petit to light for 6 hours at the strongest light setting. It will last 24 hours at the first light setting and 15 hours at the medium setting.
It's possible to leave the Edison the Petit connected with the (original) charger. However, this will make the battery "lazy". Meaning that after leaving the Edison the Petit plugged in for a long time it will no longer be able to light up as long as stated in the manual (depending on the setting 6, 15, or 24 hours).