40 CFR 27.47: Limitations.

(a) The notice of hearing with respect to a claim or statement must be served in the manner specified in § 27.8 within 6 years after the date on which such claim or statement is made.

(b) If the defendant fails to file a timely answer, service of a notice under § 27.10(b) shall be deemed a notice of hearing for purposes of this section.

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