I'm also curious as to why you want to know this, but frankly I can't resist answering in spite of not knowing. ;)
I wouldn't let go until he passed out or I was able to knock him out cold with another weapon. And I would squeeze, pull, and twist, as per the recommendation of my partner.
I usually carry a knife or I'm with my dog, so... well, it wouldn't be pretty. According to my partner, if I were able to cut a man's genitals, he would most certainly bleed to death, particularly if he were erect. My dog? Point at it and give a bite command. That's why we have bite and release commands. He is trained to not release an object until told to cease.
At this point, if I were attacked again and armed or had any advantage, I would probably not stop until I was sure he wasn't getting up, if not kill him. I live in fear that I will kill someone because of this. I'm not sure I'd stop.