Sometimes determining when a patient is crossing a line is more obvious when doing multiple surgeries than when doing fewer surgeries since there may be fewer constraints on single procedures but that might be still inappropriate for an individual patient. There are no hard and fast rules except as sometimes determined by law that might restrict the length of surgery, the amount of fat removed in liposuction, or the type of anesthesia administered in particular settings.
Crossing the line can involve psycho-emotionally inappropriate situations and also medical or physiological guidelines. One test is to ask what would happen if something went wrong. Of course if everything goes wll, no one will criticize, no problems will have occurred and everyone is happy. The problems comes when something goes wrong. Can both the patient and the surgeon justify the decisions and course of actions? If there is a sense that regret might result, you might what to rethink the decision.
A doctor must spend enough time with the patient to explore the "why's" of the request and not just accept the "what's" that should be done. Is the patient looking for improvement or running away from imperfection? How secure or insecure is the patient? An insecure patient who has a complication can end up a disaster emotionally.
Physiologically, we look at time under anesthesia since complications increase the longer one is under anesthesia, the total surface area of the surgeries since swelling and bruising over a wide area is often more physiologically taxing than surgery concentrated in a smaller region, and whether one procedure is additive in complication risks to another such as can be the case when liposuctioning an area that one is also lifting and tightening.