half-life

puoliintumisaika puhekieltä The time required for half of the nucleus nuclei in a sample of a specific isotope to undergo radioactive decay.
puhekieltä In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
puhekieltä The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
The time it takes for an idea or a fashion to lose half of its influential power.
1991, Robert Ackerman, Introduction to Jane Ellen Harrison's Prolegomena to the Study of Greek Religion (1903)

Most books of scholarship have surprisingly short intellectual 'half-lives during which they make a difference"