How long do you need to be purging for it to start causing bad side effects?

Eating disorder. It all depends on what you mean by binging and purging. It is possible for purging to result in a esophagus tear which could bleed, electrolytes like potassium are very important especially for heart rhythm could cause lethal problems. Usually these problems aren't severe the first time you purge-but they could be. Please confide your disorder to a trusted adult and a physician and seek help asap.