Some background: my kids (4 and 7) are currently obsessed with the "Ninja Kidz" a team of siblings who do gymnastic shit on youtube. My 7 year old has reaffirmed he wants to be a ninja when he grows up. I've told him many times that BJJ (which he currently trains) + basic wrestling/Judo/MMA is more effective but no, he wants to be a ninja.
These ninja kids recently did a dead hang contest and the winner did
15 minutes and was knocking out pull ups for style points when they made him stop. What the actual fuck. I know it's easier if you're lighter, but still.
We did a family dead hang contest right after dinner (DOH grip on straight bar) on my squat rack in the basement:
Me (5' 8" 170 lbs): 1:23 on straight bar with DOH grip. 1:34 on parallel grip bars an hour later.
7 yo son (~50 lbs): 50 seconds
4 yo daughter (~37 lbs): 34 seconds
wife (doesn't train): 15 seconds
For a regular sized adult, it does start to get a lot harder after around a minute. Lactic acid is building up in your forearms and it's about how long you can take the pain. I think you'd have to be a competitive rock climber or Ninja Warrior competitor or some shit to do over 5 minutes.
This dude has the world record dead hang at 1 hour 20 minutes, 41 seconds:
https://www.guinnessworldrecords.com/world-records/118967-longest-duration-in-the-dead-hang-position#:~:text=The longest duration in the,Japan, on 18 November 2022.