What are the chances of needing surgery for severe mitral valve regurgitation?

Depends on symptoms. Symptoms generally dictate need for corrective surgery (repair vs replacement) in mr cases. Consult with a cardiologist.
High. Severe mitral regurgitation typically will need surgery (hopefully repaired) at some point. Even if the patient has no symptoms eventually the will develop them. Also the heart will begin to dilate which can be a reason to fix it. Irregular heart rhythms may develop which is a reason to operate. The bottom line is severe mr is not a stable condition it will continue to progress.