time for substances to decay
Radioactive substances decay randomly. The decay is proportional to the amount there is. This results in the number of atoms decaying into other elements gradually becoming lesser and lesser as time goes by.
The process is exponential.
example
the amount of carbon-14 in a bone is halved in a time that is specific to carbon-14 which is about 5730 years. This is called the half-life of a radioactive element. For example, the activity in a bone of a dinosaur may have been about 720 Becquerel. After 5730 years, the activity would has dropped to 360 Bq. After another 5730 years, the bone’s activity reads 180 Bq. This means that after 2 half-lives (Halbwertszeiten) the activity has reduced to 1 / 4.
The formula is
$${N_t = N_0 \cdot \left(\frac{1}{2}\right)^{\frac{t}{HL}}}$$
Here, $N_0$ is the initial amount and $N_t$ is the amount after a time $t$. Also, $HL$ is the half-life of the substance in question.