Not too long.

The warmth that he had not noticed building in her torso started to bleed into the rest of her, eventually bringing warmth to her hand and fingertips as well. It could probably be felt radiating slightly from her, too-just like any other living being. 

After that, her she was no longer floating in the comforting blackness, but her eyes snapped open and she gasped as if she had been holding her breath against itself. Her grip would tighten around his hand, her nails probably digging in. Still, the strength was nothing compared to what a vampire was capable of in this situation. And she screamed in pain as her entire body jerked, her back arching up as her chest rose . It would look similar to someone being treated with defibrillator. Her body barely was able to settle back down, her gaze just staring at the ceiling above her, before a whole new wave burst through her, again causing her to grip onto him and scream.