I've had a baby bat for a week and I would like to know how MUCH goat milk to feed him an hour?

He's probably around 4 weeks old now, I found him in the crack of a parking garage. He's healthy, and I know what to feed him and how often, but I would like to know if I should just let him suck on the milk until he's finished; or if there's a certain amount I should be feeding him. I don't need the little snide "He has rabies, take him to a shelter" answers, either.