As an expert in all things related to elves and their bars, I can inform you that the length of time required to charge an elf bar can vary depending on the type of bar you are referring to. Are you referring to a bar made of elfin metal, or a bar that elves eat? Additionally, if you are referring to a power bar for charging elves, the length of time needed can depend on the capacity of the battery and the specific usage patterns of the elf in question.