For domestic, non-military voters who choose to use the mail to return their completed ballot: The Postal Service recommends that they mail their completed ballot before election day, and at least one week before the deadline by which their completed ballot must be received by their election office. Some states may recommend allowing even more time for mailing completed ballots.
All voters should keep in mind the time required for each leg of mail delivery—to the voter and back to the election office—when deciding how to participate in an election.