5 CFR § 352.207
Exercise. The time limits for application for reemployment under this subpart are:
(1) Within 30 calendar days before the expiration of the term of reemployment rights;
(2) Within 30 calendar days after receipt of notice of involuntary separation;
(3) At least 30 calendar days in advance of the person's scheduled entry into active military duty. In this case he shall be reemployed and separated, furloughed, or granted leave of absence for military service by the reemploying agency; or
(4) At any time before the expiration of the term of reemployment rights with the written consent of the current employing agency if application for reemployment is made within 30 days after date of separation, or after receipt of advance notice of proposed demotion by the current employing agency.