When should I be concerned enough about my hives to go to the er?

When severe. You should become concerned enough to go to the er if your hives are severe, or if they are associated with cough, shortness of breath, wheezing, or other breathing difficulty including tongue or throat swelling.
Other associated Sx. Even though scary looking, hives all by itself is not life threatening. U can take some antihistamine first. But if it happens right after u ate something that u may be allegic to, and/or it is associated with other symptoms, such as coughing, wheezing, throat swelling, difficulty breathing, fainting, belly pain, nausea, vomitting, etc., go to er or call 911!