How much time does it take a human body to replace all of its atoms?
Not all the atoms in the human body are replaced.

Carbon atoms in tooth enamel are not replaced after the enamel is laid down during tooth development. This was demonstrated in the 2005 Nature article "Forensics: Age written in teeth by nuclear tests", where the researchers showed that the amount of carbon-14 in tooth enamel can be used to accurately determine date of birth.