I use Bib’s guideline of 20 minutes max on and 10 minutes minimum off.
I check for color, temperature, and especially numbness.
Even if it feels ok I stop at 20 minutes, wait at least 10 minutes, and then increase the weight.
My last set usually only lasts 15 minutes or so, due to fatigue (discomfort) or mild numbness (feeling comes back in less than 10 seconds). I usually do 4 or 5 sets.
I am sure that I have not experienced any tissue death. I have more sensitivity than I had when I started.
There was a time when I hung too long and did experience some prolonged numbness and had some trouble getting erect. A little scary. I now use morning and evening wood as a guide to increasing/decreaseing the weight and/or number of sets. It seems that knowledge gained from experience is key to achieving continued gains.