The effect will not be permanent in the sense that you remove the stretcher and they stay down there....
You have two competing factors fighting you...
One is the not only elastic nature of the scrotum but the fact the scrotal skin responds to a bunch of environmental factors to help control the temperature of your nuts.
The scrotum, no matter how far you stretch, retains the ability to both shrink, and thicken... and when it does this without the stretchers on... it will pull them up almost back to normal.
The other factor fighting you is the suspensatory ligaments... these are tough ligaments that attach to each testicle and can literally pull your nuts up inside your body, both as a defense, and as a temperature control reaction. There are also arteries and veins that are only so long....
No matter how much you stretch your scrotum, its the length of the suspensatory ligaments that define how far down your balls will hang when warm.
When using stretchers, you want to add length to the point where you feel a deep pull from behind your pubic bone... not too much at a time... but a steady pull... only when you feel this are you actually stretching the ligaments.
Keep in mind the significant stretching will NOT make your balls bigger... the enlargement you think you are seeing is nothing but edema (fluid buildup) in the tissues of your scrotum.
Go without for a few days to a week and that edema will subside.
Edema is dangerous when stretching... it can cause the stretcher to become tighter...squeezing off blood flow to your testicles...
And that could damage or kill your testes outright.
Also good advice is to NOT go as far as you can as fast as you can... if you stretch too fast you can herniate the blood vessels supplying the testes... this can compromise blood flow and result in one or both testes atrophying for lack of blood--- which will make them smaller, not larger.
A key thing to realize is that stretching does not make your balls bigger... but it does cause you to grow additional scrotal skin.
So your scrotum will become larger.
SOme guys also inflate this enlarged scrotal sack with saline or pumping, which enlarges it further and makes pumps up the scrotal skin with edema.
My advice is to use weither leather wraps or stainless steel split collars to not only stretch the balls away from the body , but to add significant weight to acheive that deep ligament pull...
And take it slow. Wait until you can comfortably wear the weights pretty much all day without your nuts getting blue or cold and gradually step up in half inch increments over a couple of years.
Your balls will never hang to you knees without any weights at all...
But they WILL hang measurably lower than they did before stretching... maybe a third or quarter what you get with weights...
But if you are like me... the weights will become addictive. I wouldn't dream of having sex without 4 pounds of ball weights... and most days I wear at least 2 pounds of weights just to feel the tug.