An atom of a given isotope will undergo radioactive decay whenever it feels like it. No joke. The nucleus of a radioactive isotope is unstable. Always. But that atom has no predictable moment of instability leading immediately to the decay event. We use something called a half life to estimate how long it will take for half a given quantity of an isotope to undergo radioactive decay until half the original amount is left, but this is a statistically calculated period. No one knows how long it will take a given atom of a radioactive isotope to decay, except that those with very short half lives will pretty much disappear relatively quickly.