How often were the games who the Arrephoros worked to prepare the peplos for held?
Arrephorion:  It provided the lodgings for the Arrephoros, four noble Athenian girls who worked to prepare the peplos which would be used in the Panathenaic Games. Panathenaic Games: The Panathenaic Games were held every four years in Athens in Ancient Greece from 566 BC to the 3rd century AD.
every four years