Oh boy, here's where I sound like a psychopath… It's only meant to advance the plot, I swear!
Alrighty, so, theoretically, how long do you think a person would take to pass out after being burned repeatedly by a hot poker? Mostly centered around his torso, but a few burns on his arms and neck?
And which way, in your opinion, would be the most efficient method of portraying the pain he's in?
Depends. There are many factors to consider. First thing is their pain tolerance. If the worst pain he's ever had was stubbing his toe, chances are he won't be able to keep up for very long.
Second will depend on your character's body. Skinny/little muscle will feel more painful, but more fat/muscle will absorb the heat a little more (think the difference between burning your scalp/ear with a curling iron to grabbing a pot coming out of the oven.) It tends to be that the bigger you are, the more your body can take before you begin to shut down.
Third factor is their physical state. How much sleep they've had, what they were doing before getting tortured, their diet, etc. Adrenaline is a large part of this factor. High adrenaline, they'll be able to push through for a long while, little to no adrenaline and they will be out before the poker gets close to their skin. Similarly, a character that's been eating low calories/low energy/low protein food, their body will
Finally, age. As horrible as it is to even consider, I'm going to address it anyway. Child characters dealing with burning/fire are not going to last long, if fear doesn't get them first. Teens will depend heavily on their body type, and their motivation. If the teen is being tortured for information, they will likely stay up for a while out of sheer stubbornness, but if it's pain for pain's sake, they will likely fall quicker, since they have nothing to fight for. Adults don't have an age factor, in honesty, and mostly depend on the other three. Elderly, it's difficult to say. Especially with burning. I can't help much here, but hopefully you don't plan on torturing the elderly. I mean in your writing.
Ultimately, it's your choice on how much your character can stand. If you need the character awake longer for plot reasons, then let them stay awake. If you need them out like a light quickly, make it happen.
Portraying burning can be fun difficult, depending on what you are comfortable writing. The most important thing to remember is that we (the reader) already know that it's hot. Comparing the burns to other hot things won't do much. You can compare once, maybe twice for longer scenes, but too much will make your writing weaker.
Describe what's happening to what the poker touches. Is the character clothed? Then have your torturer hold the poker against his shirt/jacket/whatever for a few minutes; Describe the way the fabric tears and curls away, how the character can feel the the poker just millimeters away from their skin.
Is the poker sharp? Does it dig into the skin, and burn from the inside? Or is it blunt, and digs into someone? Is it hot enough to cauterize any cuts it makes?
Other effective descriptions I personally like is feeling your skin grow dry/brittle, or feeling each layer of skin burn; burning away hairs is always fun, and you can get creative with that one. Eyebrows, eyelashes (this one is also fun if you want to describe how dry their eyes are and/or how it hurts to blink and/or how they can't cry), armpit hair (under most joints tend to be very sensitive to heat: armpit, back of your knees, the undersides of your toes, etc.)
Regarding efficiency in particular, you don't want to overdo the description. Not only does it chase away readers, it also loses effectiveness. Personally, I find that multiple short descriptions in between dialogue/thoughts/action tends to be easier.
Here I've written 2 examples, using the same base dialogue/actions, but using 2 different methods to portray pain.
"I'm not telling you anything!" He cried out, his body attempting to curl away from the heat. The poker pushed into his abs, burning away each layer of skin.
I need to get out of this He thought, panting. But how?
"We have ways of making you talk." As the metal began falling towards his hips, his attacker smirked, before passing the poker to one of his colleagues.
Motherfucker! The poker was jabbed in between his ribs. He could feel the metal clanging between the bones, the pain echoing through his body each clang multiplying the horrid heat.
"Confess, and maybe we'll let you live!" One of his captors said, chuckling. [Character] groaned as the iron began prodding his neck. He gathered the horrible, metallic pain and spat on the first man's shoes.
"Only time I'll confess is at the Pearly Gates." [Character] rasped. His captors scowled.
"We'll see about that."
The air hung heavy with smoke and screams.
"I'm not telling you anything!" He cried out. Though [Character] was soaked with sweat, his skin was dry and brittle.
I need to get out of this He thought, grinding his teeth as each layer of skin shriveled away. But how?
"We have ways of making you talk." His attacker smirked, passing the poker to one of his colleagues. The iron was pressed against the sole of his foot
Motherfucker! Each flare of pain was accompanied by the scent of flesh.
"Confess, and maybe we'll let you live!" One of his captors said, chuckling.
"Only time I'll confess is at the Pearly Gates." [Character] rasped, his throat dry and cracking. His captors scowled.
"We'll see about that." His muscles broiled as the iron dug into his thighs
The smell of burnt flesh deepened as the shrieks grew louder.
^^^ Admittedly, not my best work, but it served its purpose. Both examples are efficient, but they read differently from each other.
The first example had fewer mentions of torture, but were longer. This gave a more in depth view of the pain and, depending how you write it, can make the pain echo inside you. And cool as that is, it can drive readers away.
The second example mentioned torture a lot more frequently, but were shorter. This gives a reader a sort of adrenaline rush while reading, making the stakes seem higher; and the action is happening right in front of them.
Hope this helps!