Is the expression “Where this leads” used generally?
Yes. "Where this leads" is a very common expression and is used quite frequently. "Where this leads" means the result of doing something. She is telling her son that he will eat cereal over and over if he adds more milk to it. Here are some examples of "Where this leads" being used: Ex: We know where this will lead if I let you eat in front of the television. Ex: I can't let you eat candy for breakfast. We know where this leads: obesity. Ex: Everyone knows where long-term smoking leads: cancer. Thank you for your question!