29 CFR 580.5: Finality of notice.

If the person charged with violations does not, within 15 days after receipt of the notice, take exception to the determination that the violation or violations for which the penalty is imposed occurred, the administrative determination by the Administrator of the amount of such penalty shall be deemed final and not subject to administrative or judicial review. Upon the determination becoming final in such a manner, collection and recovery of the penalty shall be instituted pursuant to § 580.18.