34 C.F.R. § 300.303
(a) General. A public agency must ensure that a reevaluation of each child with a disability is conducted in accordance with §§ 300.304 through 300.311—
(b) Limitation. A reevaluation conducted under paragraph (a) of this section—
(2) Must occur at least once every 3 years, unless the parent and the public agency agree that a reevaluation is unnecessary.
(Authority: 20 U.S.C. 1414(a)(2))