One of the Wizard's passive abilities is called Illusionist:
Whenever you suffer more than 15% of your Life in a single hit, the cooldowns on Mirror Image and Teleport are automatically reset.
Is this 15% of your max life or current life?
Although I didn't find anything to support this, logic and compare with other skills tells me it's total life. If it was about current life, it would have said 15% of your current Life.
Also, if it was about 15% of current life, it wouldn't have been very ok in terms of gameplay. Think about it, after you drop below 50% health, every time you get a hit of 15%(that's 7.5% if I'm not mistaken) you reset your cooldown. That's almost like reset cooldown on every slightly above average hit. When you go below 25 of health most probably every hit will reset your cooldown if we follow the same principle(it's ok to presume that most hits(even mob hits) will take about 3% of your life if you are in an area corresponding to your level).
So as I stated above, documentation didn't provide full info, but taking logic and previous Blizzard skills even from different games in consideration, we can asume it talks of 15% of your maximum life.