Witness Reverend Jeff Hood told reporters he saw a man ‘struggling for their life’ for 22 minutes as Smith became the first US death row inmate executed by nitrogen asphyxia
Which says to me that from the time they brought him in and strapped him down until he died lasted about 22 minutes and the murderer struggled physically against the restraints the entire time.
This quote farther down suggests from the time they started administering the gas until he died only took a couple of minutes:
But, witnesses said Smith appeared conscious for several minutes, shaking and writhing on the gurney.
Several could be 25, and he could have been shaking from pain and agony, but it seems more likely he was holding his breath and shaking out of fear while trying to fight and get free.
Keep in mind that the first quote is from his anti-death penalty spiritual advisor and this entire article is brought to us by a magazine with an "end the death penalty campaign".
I'm generally anti-death penalty myself, but nitrogen asphyxiation seems way better than electrocution, lethal injection, or hanging. They could probably do it better by using some kind of general anesthesia to render him unconscious and then flood the room with pure nitrogen, or even just get rid if the death penalty all together. Unfortunately this is the world we live in and so fae this is the least bad option we've seen.