What is the half life of the penny

Back about thirty years ago, I took a huge container full of pennies and, with nothing better to do, sorted them by year and stacked them. I found that, yes, the distribution is a pure exponential decay, for a good thirty or forty years. I determined that the size of a year's stack 7 years back from a later stack is about half the size.
Thus, the half life of a penny is 7 years.