How dangerous is prolonged tachycardia for the heart?

This is unlikely to lead to any problems, especially if it is intermittent and you are otherwise healthy. A healthy heart can easily sustain a heart rate of 100-120 for this period of time. If a person has an underlying heart problem, such as congestive heart failure, then they could develop worsening heart failure at this heart rate. There is a condition known as tachycardia induced cardiomyopathy in which the heart's function can worsen secondary to a persistent tachycardia usually ociated with an abnormal heart rhythm (e.g. atrial fibrillation.) Controlling the heart rate would reverse the heart dysfunction and the heart should return back to normal. I am uming your tachycardia is secondary to a normal fast rhythm (sinus tachycardia) and this is not a known cause of tachycardia induced cardiomyopathy.