Let's calculate how much the shield absorbs in practice. To quote developer Samhayne's post 5/8/2013 in the sorcerer forum: "Without upgrade to time + extra damage: (((2*strength)+intelligence)*3)

With upgrade to time + extra damage: (((2*strength)+intelligence)*3.5) plus the shield lasts an extra 5 seconds."

The "upgrade" Samhayne mentions is the "Extended Shield" upgrade that increases the amount of damage absorbed by 15% (3*15%=.45 which is rounded to .5 in the formula above.) Now we have Shield Mastery which also increases the amount of damage absorbed by 15%. So if we assume it adds another .5 to the formula we get:
With upgrade to time + extra damage+mastery: (((2*strength)+intelligence)*4.0).

So lets assume a geared lvl 66 mage has 1,200 INT and 400 STR. The maximum reduced damage absorbed would be (((2x400)+1200)*4.0) = 8,000. However, a charged arcane shield reduces damage by 55%. Therefore, the amount of damage prior to shield reduction needed to break shield is 8,000/45%=17,777 damage. Now lets assume armor further reduces damage by 50%. Therefore total unreduced damage to break shield would be 17,777/50%=35,554. Question: how long does it take to do 35,554 damage to a naked mage (naked is unreduced)? That would begin to explain whether or not the arcane shield formula needs to be updated.

Disclaimer: I don't have a 66 so i am just guessing as far as STR/INT and armor reduction. If someone has a better grasp on 66 stats it would be much appreciated to do the same analysis to come up with a number for unmitigated damage required to break shield. I apologize if my math is wrong haha, but lets work on that together.