20 CFR §404.1298

1. (a)Time for filing. The State must file the civil action for a redetermination of the correctness of the assessment, allowance or disallowance within 2 years from the date the Commissioner mails to the State the notice of the decision under § 404.1296. Where the 2-year period ends on a Saturday, Sunday, legal holiday or Federal nonwork day, an action filed on the next Federal workday is considered timely filed.
2. (b)Extension of time for filing. The Commissioner, for good cause shown, may upon written application by a State filed prior to the end of the two-year period, extend the time for filing the civil action.