How long of not treting gerd can it cause harm to my esophagus?

No short answer: It depends on how long and well you treated it before you stopped. If you put 2 bricks under each side of the head of your bed it will let gravity help keep the acid out of the esophagus. Also carafate (sucralfate) (sulcrafate) liquid in the evening or 3 times a day may help more cheaply. Zantac helps some people. If you're having heartburn you're causing damage. Some people have damage without heartburn.