cuddlefish

The way exponential decay works is the decay rate (how fast the atoms are "converting" is the best way to put it, the actual mechanics of that are... complicated) is really fast but slows down the less stuff there is. You start with a very rapid decrease in amount of your sample at the beginning, but one of the properties of an exponential curve is that it doesn't ever ever every go to zero (until you extend time to infinity). However, for all our purposes at some point the radiation becomes so small it won't even register in our detectors.