It depends upon the level of enemy damaging you and how much they attempt to damage you that will give you a percentage back. If you just have dregs/psions shooting you, you'll get next to nothing. A boss, a good bit. This is pretty much my main class I run and they do work just fine and as intended. Also note, if you are in Crucible and someone hits you with a Disruptor void perk, after a few shots it will knock you out of your super with nothing regained.