34 C.F.R.  §300.536.  Reevaluation.

Each public agency shall ensure--
(a) That the IEP of each child with a disability is reviewed in accordance with Secs. 300.340-300.350; and
(b) That a reevaluation of each child, in accordance with Secs. 300.532-300.535, is conducted if conditions warrant a reevaluation, or if the child's parent or teacher requests a reevaluation, but at least once every three years.