14 CFR § 121.521 - Flight time limitations: Crew of two pilots and one additional airman as required.
(a) No certificate holder conducting supplemental operations may schedule an airman to be aloft as a member of the flight crew in an airplane that has a crew of two pilots and at least one additional flight crewmember for more than 12 hours during any 24 consecutive hours.
(b) If an airman has been aloft as a member of a flight crew for 20 or more hours during any 48 consecutive hours or 24 or more hours during any 72 consecutive hours, he must be given at least 18 hours of rest before being assigned to any duty with the certificate holder. In any case, he must be relieved of all duty for at least 24 consecutive hours during any seven consecutive days.
(c) No airman may be aloft as a flight crewmember more than -
(1) 120 hours during any 30 consecutive days; or
(2) 300 hours during any 90 consecutive days.