There are two kinds of people in this world: the kind who notice that the popping of popcorn plotted over time follows a Gaussian distribution, and the kind who don't. Almost every package I have seen says to wait until the pops come 1 second apart. This must be some kind of 2σ point. I can imagine popcorn "scientists" working with stopwatches and white lab coats, measuring the average popping time of different popcorn kernals in different microwave ovens. And it must be a common finding that ~95% of the corn is popped when the popping frequency is at one per second for the usual bag size.