The time it takes for half of a radioactive sample to decay is called its half-life.
Half-life is the time required for half the unstable nuclei in a sample to undergo decay. It is a statistical property of a radionuclide: no one can predict exactly when a particular atom will decay, but a sufficiently large population follows an exponential decay law. After two half-lives, one-quarter of the original radioactive nuclei remain; after three, one-eighth remain.
The half-life is related to the decay constant by t₁/₂ = ln(2)/λ. Different isotopes have dramatically different half-lives, from fractions of a second to times far exceeding the age of Earth. Carbon-14, with a half-life of about 5,730 years, is useful for dating once-living material.
Half-life is commonly confused with mean life and decay constant. Mean life is the average lifetime and equals 1/λ for a simple decay process; it is longer than the half-life by a factor of about 1.443. Activity instead measures the number of decays occurring per unit time, not the time for the sample to halve.