Eligibility and evaluations for special education services are governed by the Individuals with Disabilities Education Act of 2004, Section 1414 (the full text of 20 U.S.C. Section 1414 related to eligibility and evaluations is below the fold).
Under IDEA a parent can request an initial evaluation for special education services at any time. In addition a school can recommend and request consent for an initial evaluation at any time. Once a request has been made and a parent has consented to evaluation a district has 60 calendar days to complete the evaluation. It is important to note that parental consent to an evaluation is not consent for services.
If your child has previously been evaluated, you are entitled to demand a reevaluation once a year.