Found it, I was close; 3 pilots= 18hr duty with 12hr flight time 4 pilot= 20hr duty with 16hr flight time.
You can see it in FAR 135.269
Which I will reprint for your reading pleasure!
Title 14: Aeronautics and Space
PART 135—OPERATING REQUIREMENTS: COMMUTER AND ON DEMAND OPERATIONS AND RULES GOVERNING PERSONS ON BOARD SUCH AIRCRAFT
Subpart F—Crewmember Flight Time and Duty Period Limitations and Rest Requirements
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§ 135.269 Flight time limitations and rest requirements: Unscheduled three- and four-pilot crews.
(a) No certificate holder may assign any flight crewmember, and no flight crewmember may accept an assignment, for flight time as a member of a three- or four-pilot crew if that crewmember's total flight time in all commercial flying will exceed—
(1) 500 hours in any calendar quarter.
(2) 800 hours in any two consecutive calendar quarters.
(3) 1,400 hours in any calendar year.
(b) No certificate holder may assign any pilot to a crew of three or four pilots, unless that assignment provides—
(1) At least 10 consecutive hours of rest immediately preceding the assignment;
(2) No more than 8 hours of flight deck duty in any 24 consecutive hours;
(3) No more than 18 duty hours for a three-pilot crew or 20 duty hours for a four-pilot crew in any 24 consecutive hours;
(4) No more than 12 hours aloft for a three-pilot crew or 16 hours aloft for a four-pilot crew during the maximum duty hours specified in paragraph (b)(3) of this section;
(5) Adequate sleeping facilities on the aircraft for the relief pilot;
(6) Upon completion of the assignment, a rest period of at least 12 hours;
(7) For a three-pilot crew, a crew which consists of at least the following:
(i) A pilot in command (PIC) who meets the applicable flight crewmember requirements of subpart E of part 135;
(ii) A PIC who meets the applicable flight crewmember requirements of subpart E of part 135, except those prescribed in §§135.244 and 135.247; and
(iii) A second in command (SIC) who meets the SIC qualifications of §135.245.
For a four-pilot crew, at least three pilots who meet the conditions of paragraph (b)(7) of this section, plus a fourth pilot who meets the SIC qualifications of §135.245.
(c) When a flight crewmember has exceeded the daily flight deck duty limitation in this section by more than 60 minutes, because of circumstances beyond the control of the certificate holder or flight crewmember, that flight crewmember must have a rest period before the next duty period of at least 16 consecutive hours.
(d) A certificate holder must provide each flight crewmember at least 13 rest periods of at least 24 consecutive hours each in each calendar quarter.