What is the amount of time required for half of a sample of radioactive isotope to break down and form a daughter isotope?
The amount of time is called a half life. The length of time of a half life varies with the element.
Urainium 238 has a half life of 4.5 billion years while Carbon 14 has a half life of 5,700 years. In both cases the half life is the length of time required for 50% of the parent element to decay into the daughter element.