20 C.F.R. § 404.1286
(a) Subject to the exceptions to the time limitations in §§ 404.1287 and 404.1289, a State is not liable for an amount due under an agreement unless the Commissioner makes an assessment for that amount before the later of the following periods ends:
[53 FR 32976, Aug. 29, 1988, as amended at 62 FR 38451, July 18, 1997]