Cereal in milk

When did you start cereal in babies milk? My baby is 3 1/2 months she is a very big baby. She just ate 12 ounces of milk. I would feed her 4 ounces then burp her and she would scream until I got her more milk until she was finally satisfied. She has an appointment January 22nd. So I know I should ask her dr then. But I just want to see other opinions. Thank you