Final answer:
In Aeneid 6, the Sibyl states that unburied souls must roam for a period of 1,000 years before crossing the river Styx, which is based on the myth of Er and associated with their purification process.
Step-by-step explanation:
According to the Sibyl in Aeneid 6, the souls of the unburied must roam for 1,000 years before they are eligible to cross over the Styx. This period of time is mentioned as mille rotam volvere per annos, indicating a cycle or rotation of a thousand years that the souls must undergo as part of their purification process.
The concept is said to be inspired by the myth of Er from Plato, which Virgil likely adapted into his work. In this myth, souls must drink from the river Lethe to forget their mortal lives before reincarnation, which further elaborates on the journey of the soul in the afterlife within the context of Orphic and Greek eschatological beliefs.