How long does it take for the human body to fully regenerate all of its cells?

I've heard people say it takes 6 months/7 years/15 and a half years to fully regenerate every single one of your cells. What is the real length of time it takes? source please.
Update: What I mean is, how long does it take for all of your cells from a specific time be diminished from your body with new ones taking their place? How long does it take for you to become a totally different human being with new cells?
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