I just started to do these- I’m not doing normal stretches anymore. Normal stretches seemed only to stretch my skin, making my pubes creep up my shaft.
About how long you should do these- I find that it takes about 10 seconds for me to fully relax into the stretch, and actually get any benefit from it, but obviously you should see what works for you.
About how many you should do, I started with 4x20 seconds, but I think if you can keep a good grip, it’s probably a good idea to hold the stretch for a bit longer. I could be wrong, this is just what appears to be working for me.
Experiment!
Good luck.