How low does the average males testicles hang?

Everyone's different. And it also depends if the weather is hot or cold. naturevalleybar posted pics, and these are absolutely spot-on just right for cool-ish temperatures.

Some guys' balls hang really low (for example, look for films with Barrett Long). Some girls (and guys) love this, but some really don't and prefer them like naturevalleybar's perfectly decent balls.

I started improvising and making my own ballstretchers from when I was about 12. Then when I had money (and realised that they existed) I started to buy pre-made leather ones. In winter they're just like naturevalleybar's. But in summer, after 15 years of training, they hang really low (like Barrett Long) and are like a pendulum in a grandfather clock and can be twisted and pulled without any pain at all.

But be careful before you start training your balls to stretch - they might feel good inside your boxers in summer, but not everyone likes the look.
 
I contacted online doctors regarding a scrotum lift, mine are hanging 6" away from my crotch, given my height that is quite close to "halfway down my knees"
From what they told me from the pictures i sent, I was a good candidate because my testicles were hanging below my soft penis. which is also 6" soft.
So if a "normal" scrotum should hang above a soft "normal" penis (4 inches?) I'd assume that the average testicle hang is somewhere around 3"