Anything over 15 hours flight is a health hazard. Ask the ex AN pilots who flew the SQ A340-500 SIN-EWR-SIN. In fact one of them is being buried in MEL on Tuesday 18 Feb.
It is generally accepted that a single cigarette reduces ones life between 7 and 11 minutes. I would like to know the reduction in life for one ULH flight. We know the radiation dose at FL350 is 0.003 millisievirts per hour. There are other health hazards in addition to radiation for long duty periods.