In my state, we are legally required to keep medical records 10 years from the last visit. After that, they can be destroyed.
I have absolutely no idea if this is a state law or federal. In TN, we only had to keep them 7 years, but that was almost 10 years ago, so the law may have changed.
It may vary from specialty to specialty, also! Up until the last decade, my FIL's widow was still getting requests from patients he had treated in the 50s and 60s! He was OB/GYN, and back then, DES, which has resulted in problems in the babies that did not show up until ADULTHOOD, was given to prevent miscarriages. She had all of his records until a few years ago, so it may be that you have to keep them until 10 years after the doctor's death in some states.