29 C.F.R. § 4003.4

When a document is required under this part to be filed within a prescribed period of time, an extension of time to file will be granted only upon good cause shown and only when the request for an extension is made before the expiration of the time prescribed. The request for an extension must be in writing and state why additional time is needed and the amount of additional time requested. The filing of a request for an extension will stop the running of the prescribed period of time. Requests for extension of the time to submit an appeal should be sent to the Appeals Board; requests for extension of the time to submit a request for reconsideration should be sent to the department that issued the initial determination. When a request for an extension is granted, PBGC will notify the person requesting the extension, in writing, of the amount of additional time granted. When a request for an extension is denied, PBGC will notify the person requesting the extension in writing, and the prescribed period of time will resume running from the date of denial.