[@The Mad Hatter]
Hmm, I barely passed physics, so I didn't think about it too much. I was planning on just going with what felt balanced.

Going by her backstory as it is now, though, they can block smaller caliber bullets and shrapnel head on, so it takes a pretty high amount of pressure to break through. It's worth mentioning that she could also angle her barriers, to deflect things instead of trying to outright stop them.

The actual durability of a barrier is not very high. Once it blocks an attack or two, even weaker ones, it'll start to 'crack' and lose its integrity. Right now, I see the character creating one shield for one specific attack against her, and then it either shattering or her dispelling it immediately before creating a new one. The idea is that it's more of a finesse-based and reactionary power.

If that seems like too much, I can easily tone some things down.