The validity period of a SAFc is how long it is eligible for retirement before it automatically expires in the registry. SAFc and any SAFcA/SAFcE that are unbundled from it are valid for 24 months after the initial SAFc's issuance (i.e., all SAFcA and SAFcE inherit the expiration date from their parent SAFc).
At the end of its validity period, a SAFc "expires" and can no longer be used.