I know its not part 91.... but when I flew Hercs our standard crew day was 16 hours. AC could request up to a 2 hour extension. Beyond that required a Numbered airforce senior duty controller to coordinate. Once on an exercise, in 1989 we had our crew day extended to 22 hours due to wx and a diplomatic snafu with a foreign govt. One other time while flying a NCA mission we were extended to 25 hours..note: that on these missions we had 1 extra FE, NAV, and pilot.

Straight 91..... none. Our company POLICY is to use 135 as a guide.