We define half-life as the rate of radioactive decay. The half-life is how long it takes half the material you have to decay. The half life of a substance might be a few seconds or a few years. Since Carbon 14 has a half-life of more than 5,000 years, it's easy for us to date how old something is.
Other things have half-lives as well.
The sales cycle of the typical popular book, for example, has a half-life of about two months After two months, the book will probably have made half of all the sales it will make in its entire lifetime. For an internet viral sensation, the half-life is probably closer to six hours, measured from the moment the traffic peaks.
Every once in a while, something in the media violates this rule. To Kill a Mockingbird, for example, has a half-life of perhaps fifty years.
The art is deciding whether or not your project has hit a natural peak or whether new investment and energy can boost it to a new energy level...