If you vomit so much, when do you need to go to the hospital for dehydration?

When lightheaded. Prolonged vomiting can lead to severe dehydration, which is typically manifested by severe fatigue and lightheadedness when getting standing. If you develop these symptoms and you still cannot keep fluids down, you probably need to the hospital for intraveous fluids and other therapy to treat the vomiting. High grade fevers should also prompt early vists to the er.