It's also somewhat relative
Of course, intensity is 100% relative.
But to what?
Balls to the wall is intense for everyone and no one can do it for hours, even double digit minutes.
Hit the bag as fast and as hard as you can non-stop and IF you can do 5 mins you will be lucky.
That is, IF YOU DO IT that way.
I've seen people's idea of intense and it is pure, unadulterated, ****.
People naturally pace themselves, they can't help, and those are the ones that get so little of of those so called "intense" workouts.
If you can talk while training it is NOT intense.
If you don't have to stop to breathe, it is NOT intense.
People speak of Tabata's but very few have ever truly done them.
People speak of HIIT, but very few every do it.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !