Reaccreditation efforts should begin approximately 9-12 months prior to accreditation expiration. Formal milestones vary for each board, and can be referenced here:
ACCME: http://www.accme.org/cme-providers/reaccreditation
ACPE: https://www.acpe-accredit.org/ceproviders/application.asp
ANCC: http://www.nursecredentialing.org/Accreditation/Primary