 Approximately 75 years pass during the course of the novel, but because large amounts of this time are spent traveling close to the speed of light, the 75-year measurement is an expression of the time elapsed on Earth rather than time seen from his perspective.At one point, he estimates his natural life span to be around 250 years, but this figure is not expressed with certainty.He acknowledges that such a long life span should not be expected as a result of a mere three generations of selective breeding, but offers no alternative explanation except by having a character declare, "A mutation, of course-—which simply says that we don't know".