Like every genetically altered human, Susan had her limits when it came to using her abilities. Currently, she had held her shield long enough to affect her physical well-being. The only sign had been a nose-bleed, but the act itself left her worn for awhile. Needless to say, as long as Tony and Susan were down here the shield was staying up. They needed light and protection, but Susan also guessed, like Tony, that whatever mumbo jumbo was at work down here couldn't penetrate her little bubble.
"A while. Don't worry. I've participated in numerous tests involving my durability and found time and again to be okay for substantial periods of time", she reassured him.