"Fatigue" doesn't always accompany soreness. It's more of a state of condition than a feel. Although you can have symptoms of soreness, but that depends on the level of your training, and the plateaus that you reach.
If you base everything on the feel you get from ligament soreness then you have not properly defined "fatigue". Obviously as you progress into heavier more intense training you can feel the following symptoms......
#1 - Soreness at the base of your penis, since your ligament is attached to your pubic bone you should feel it more internally behind the fat pad.
#2 - Mild tenderness within your penis shaft. I wouldn't classify this as soreness, but more like muscle or tissue awareness. Meaning if you massage or squeeze your penis you should feel like it's been used or worked.
#3 - Soreness in you sphincter muscle. Occasionally as you graduate to heavier weight the initial sessions can manifest a soreness in your sphincter muscle. This means your body is reacting to the stress of the workload and tensing up. This is where you really need to learn to relax that pelvic floor muscle, and feel that stretch to it's fullest.
#4 - A very loose and pliable flaccid penis. This is the condition you should experience after a good hang session. It's also the time to take full advantage of this "fatigue" and do 10 - 15 minutes of very aggressive hand stretches. The hang work is what facilitates the "fatigue" and it's where you'll be able to stretch, twist and bend your penis beyond what you can normally do prior to a weight hanging session. It's in this condition that you probably wouldn't be able to achieve a 100% hard erection. This is FATIGUE!
It's my experienced that erect length gains come from flaccid length gains. Your penis can be stretched to greater lengths while flaccid than while erect.
You'll never hear anyone say that they have a 7" flaccid penis, but only a 6" erection. Over time the difference between your flaccid length and erect length will become less. For me if I had to put a percentage on it, it would be approximately 30%
If you are looking to target the ligament stretch more intensely without going to heavier weight, then use a Fulcrum bar as close to the base area as possible, all the way up to the point where you have to push your testicles backwards. Only do not exceed 15 minutes of Fulcrum hanging, as it will cut off circulation to a point. You can do sets of Fulcrums in between SD hanging.