The radioactive decay is expressed in units called becquerels ... This differs for each radioelement, ranging from fractions of a second to billions of years. For example, the half-life of Iodine 131 ...

Radioactive decay is a spontaneous and random process ... but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed. This is called the half-life.

It should also be possible to state how much of a sample remains or what the activity or count should become after a given length of time. This could be stated as a fraction, decimal or ratio.