not sure, but I think it’s cap is 100%. as long as you are holding down the attack button, you have Hyper Armor. if you stop to move out of danger or to change your position for some reason, you lose the HA until you start attacking again.
not sure if there is a difference between using a slow skill vs. a fast skill.
in addition to this quote, I read that skills using APS or Cooldowns under 1 second are necessary for Hyper Armor to work properly. you could try the test that @MpKTO suggested.
there’s a bunch of other posts, but I only read a few to get a feel for how HA works. hope this helps.