To answer lady-Rc's question: a long time, if the ropes aren't too tight. Maybe if you make the sub tie himself up, he won't over do it; he'll stop short of dangerous pain.
Obviously one can 'castrate' with something tight around the testicles: the "elastrator" is an animal castration rubber band. After a couple days the testes turn black and fall off.
That said, I've been doing bondage for 30 years and mine are still okay. However I tie myself up and don't cause myself any pain, just enough tying to keep the testicles in the sac when the scrotum is pulled. A noose can keep pulling and tighten too much if the domme is doing the pulling or the sub is being suspended. However imprisoning the testes with a commercial metal ring or with a leather/nylon strap will not increase the pressure on the cords and cause damage. I like the feel of a leather strap around the top of my scrotum, and sometimes wear it all day, even to work. Some persons use a 'parachute.' I use just a 1" dog collar, either leather or heavy nylon. There's a built in d ring to attach a thong similar to a rein. I like to think of being led around like a horse with my "ball-bridle."
For rope tying I suggest a "bottle knot." Google it to find instructions on how to tie. It is designed to go around the top of a water bottle, then with the 4 long strings tied together, the bottle is suspended from the shoulder. Tied around the top of the scrotum, the knot constricts the scrotum and the testicles are below the knot. Then the 4 long strings can be used to pull on the testicles/scrotum/cords, either with weights, by hand or whatever, while there is no further constriction around the cords that a noose would cause.
With either the collar (which I call a ball-bridle)I can take a gallon of water or 15# dumbell. I'm not brave enough to hang myself.
Bratwurst