Vlad Tepes the Impaler did thousands of impalements, depending on the stamina of the victim, some lasted 3 days. Hanging by the arms with legs tied--immobilized is much like crucifixtion. Death usually comes by suffocation as the person is too weak to exhale: 3-6 days depending on the person's condition. Some crucified persons were also stabbed in the torso to hurry death by blood loss.
Carrion birds would wait til the person is too weak to move. Crows and ravens would start with the soft tissues, usually the eyes go first.
If you want animal consumption while alive: rats and wild dogs would be the more horrendous. Depending on the latter, they may or may not go for the throat first. That could be stopped by the simple use of a protective metal or heavy leather collar. Then the dogs would go for the underbelly and the face. Rats go for openings and small parts first: various orifices, nose, ears, fingers, toes, genitals.
Correcting Poohcat: lack of water kills in 7-10 days. Dehydration causes weakness, hallucinations, dizziness, lethargy, the organs shut down: kidneys & liver, & GI tract, bp drops, blood volume drops, blood pressure and heart rate increase to compensate. Loss of only 15% of bodily water usually kills. Starvation takes 6-8 weeks.