This is a really good question and I’m afraid I don’t know the answer. I know that other transplant organs don’t last forever so I guess the same is true of the heart.
Looking at this webpage http://www.heart-transplant.co.uk/faqs.html 50-60% patients with a transplanted heart are still alive for 10 years so at least half of the hearts last for more than 10 years. (And of the patients that die in less than 10 years, many probably die because their bodies reject the heart.)
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