The half life is the time for half of the given metarial to morph through a decay process into some other substance. The actual halflife time depends on the substance's isotope. Some halflives are in fractions of a second while other may be in days, weeks, months, years, or millions of years. A halflife is a proability, A single atom in a sample has a probability of decaying. In a given halflife the odds are 50/50 that any given atom will decay into the daughter isotope by the time the halflife is up.