10 C.F.R. § 1013.47

(a) A notice to the person alleged to be liable with respect to a claim or statement shall be mailed or delivered in accordance with § 1013.8 of this part not later than the later of—

(1) 6 years after the date on which the violation of § 1013.3 is committed; or

(2) 3 years after the date on which facts material to the action are known or reasonably should have been known by the authority head, but in no event more than 10 years after the date on which the violation is committed.

(b) If the defendant fails to file a timely answer, service of notice under § 1013.10(b) of this part shall be deemed a notice of a hearing for purposes of this section.

(c) The statute of limitations may be extended by agreement of the parties.