Three or four hours. A half hour or so to load in, and hour of hair and make up/set lighting, an hour on set with talent, and add one hour minimum for contingency. Add several hours for problems: talent delays, talent issues, location issues, etc.
If all goes well and the stars align just right, you can shoot four gonzo scenes a day. That is pretty rare though, two or three is more likely.
POV bj's are obviously much less time, and you can knock them out before or after the sex scenes. You should still budget an extra hour for each one though.