5 CFR §352.312

An employee may apply for reemployment, in writing, either before or after separation from the international organization. If the employee applies before separation, the 30-day period prescribed in § 352.311 begins either with the date of the application or 30 days before the employee's date of separation from the international organization, whichever is later. If the employee applies for reemployment after separation, the application must be received by the employing agency no later than 90 days after separation from the international organization.