The Short Version
The claim reflects a common clinical shorthand, but the evidence does not support 24–72 hours as a single definitive normal range for all humans. Multiple credible sources accept about 72 hours as a normal upper limit, yet research sources report normal whole-gut transit can be well under 24 hours and varies by test method and population. The practical takeaway is that 24–72 hours is an approximation, not a universal standard.