I recommend the rent of this documentary: http://www.blockbuster.com/movies/how-jesus-died-the-final-18-hours.html
I've often seen it for free on the History Channel. They do forensic re-creation of what happened to Jesus in the last 18 hours. Short of actually killing a human being the forensic investigation is quite interesting and convincing.
Any way you look at it, Roman crucifiction was a terrible method of execution.
You also have to consider that the Romans executed the crucifiction in different postures and affixed the victims using a variety of means. Bored Romans were not beyond sport when it came to the way they handled their victims. So obviously the way in which the victim was crucified would have an impact on their duration to death.
Same would be true how the Nazis attached their victims in their morbid re-creations of crucifications. And since I'm sure their victims were in poor physical shape to begin with. That would certainly skew the results.