34 C.F.R. § 33.47

(a) The notice of hearing with respect to a claim or statement must be served in the manner specified in § 33.8 within six years after the date on which the claim or statement is made.

(b) If the defendant fails to file a timely answer, service of a notice under § 33.10(b) is deemed a notice of hearing for purposes of this section.

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