Although most natural potassium nuclei are stable 39K (93.3%) or 41K (6.7%) nuclei, about 0.0117% are radioactive 40K nuclei, which have a half-life of 1.26 billion years. If you were worried about 40K radioactivity and wanted to wait for 97 % of the 40K in the environment to decay away, how long would you have to wait?