Thank you so much ccfheartmd11 for your response. Helped alot,good to know.
Unofrtunately, the actual rate and duration of tachycardia needed to cause a cardiomyopathy is unknown. Certainly, the longer a patient is tachycardic and the faster the rate, the more likely they are to develop cardiomyopathy, but there is not a cutoff at which it becomes "most likely" or "certain".
The important thing is to control the heart rate, and in some cases also control the rhythm. This can often improve heart function. If you develop a cardiomyopathy, you should also be started on a good heart failure regimen to help prevent further negative remodeling (such as a beta blocker and ACE inhibitor).