A question: Have you seen a doctor about your high blood pressure? It's NOT going to go away! You must see a doctor ASAP. If your blood pressure is really high, you will be put on medication immediately. There's a guideline for the doctors to follow in diagnosing high blood pressure. It consists of taking several blood pressure readings over a period of a few weeks. Sometimes there's simply no time for that. If the blood pressure is really high, there's an immediate risk of end organ damage.
To answer your question: let's hope your blood pressure is not high enough to warrant a visit to the ER! Most doctors will not answer this question. Damage can occur at very high, high, or lower readings. Same with cardiovascular events such as a heart attack or a stroke. Some of the damage, such as atherosclerosis, happens over time. Everyone is different. I asked this same question several different doctors. None gave an answer. There's no good answer. One of the posters here said that, according to her doctor, if you diastolic stays at 120 for 1/2 hour (that's the second number), you should go. I went to ER several times with BP much higher than that. By then I could barely function with pressure in the neighborhood of 240/150 or higher. I would not recommend to others to wait that long. Your body usually tells you when something is wrong. Listen to it. If you feel sick, go to ER. If you are not under doctor's care right now, you must do so without delay. A trip to the ER is NOT a solution. All they do is bring your BP down to safer levels and send you home. Of course, without the meds the blood pressure goes right back up again.
Good luck and keep in touch.