5 C.F.R. § 845.204
(a) Notice. Except as provided in § 845.205, OPM will, before starting collection, tell the debtor in writing—
(b) Requests for reconsideration, waiver, and/or compromise.
(1) If a request for reconsideration, waiver, and/or compromise is returned to OPM by mail, it must be postmarked within 30 calendar days of the date of the notice detailed in paragraph (a) of this section. If a request for reconsideration, waiver, and/or compromise is hand delivered, it must be received within 30 calendar days of the date of the notice detailed in paragraph (a) of this section. OPM may extend the 30-day time limit for filing when individuals can prove that they—
(c) Reconsideration, waiver, and/or compromise decisions.