Half-Life

Half-life is constant for a given isotope and is unaffected by temperature, pressure or the amount of material.

After $n$ half-lives, the fraction of the original activity remaining is $(1/2)^n$.

Half-lives	Fraction remaining
1	1/2
2	1/4
3	1/8
4	1/16

Half-lives range from fractions of a second (for some artificial isotopes) to billions of years (e.g. uranium-238: 4.5 billion years).