Rose props herself up in a mirror image of him. She grins at his words, the tip of her tongue between her teeth. If nothing else at least the Doctor has given her parts of himself that he doesn't normally give anyone else and no matter how much it hurts later, she'll be grateful for that.

"'Least a couple of regenerations, m'guessing." She's being cautious because she'd hate for him to have to correct her and tell her 'no not quite that long'.