Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather, when a very negligible amount is still left.
Carbon dating makes use of the principle of merrydating comaccount
The half-life is always the same regardless of how many nuclei you have left, and this very useful property lies at the heart of radiocarbon dating. The graph below shows the decay curve (you may recognize it as an exponential decay) and it shows the amount, or percent, of carbon-14 remaining.
You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.
This technique works well for materials up to around 50,000 years old.
Each radioactive isotope decays by a fixed amount, and this amount is called the half-life.
Radiocarbon dating has been used extensively since its discovery.