Not sure about the toughen up power. You have to stand there for a period to do it. I presume it's like the wizard having to recharge - except they coded it that if you hold down the recharge button after using it for another attack, the wizard will start recharging, while the dwarf ignores the button if it was also held down during another attack. Which is annoying.
Also it's duration - I really have no idea. The visual indicator is so subtle it's really impossible to tell amongst the leaping and flying bodies and beatings when it ran out. Still, it's something you can do to prep before a battle and if you are doing really well, do so inside a battle as well!