If her eyes are fully open and her ears are full upright she's a pup so at least 15 days old. She may well have still been nursing though but will probably adjust to a water bottle and food fairly quickly. Personally I'd pop back around to the pet shop and pick up a second cute one of the same gender for a buddy. Rats hate to live alone, no matter how much attention you give them and can get so upset at being alone they sicken.