The half-life of any substance is the amount of time taken for half of the original quantity of the substance present to decay. The half-life of a radioactive substance is characteristic to itself, and it may be millions of years long or it may be just a few seconds.
In order to determine the half-life of a substance, we simply use:
t(1/2) = ln(2) / λ
Where λ is the decay constant for that specific isotope.