Whenever Angelic Aura is used, there's a 1-second guardian buff that repeats 5 times, making it last... 5 seconds
If you 'take damage' while it's in use, the duration of the buff decreases. There's no way of telling how much time you have left.
What if the shield meter is temporary changed when the skill is used and slowly decreases until it empties?
http://i.imgur.com/MkIC3Vq.png
1st picture - Normal
2nd - Angelic Aura used
3rd - Emptying
4th - Back to normal
This way, you can tell when the aura dies out.
+1 Because it sounds like a good idea.