Half-life: how fast stuff fizzles out. For radioactive material, it's science. For your big idea, it's a ticking clock.
Viral memes? Six hours. Books? Two months. Unless you're To Kill a Mockingbird, which is apparently radioactive in its own right with a 60-year glow.
The trick is knowing: is your project naturally winding down, or does it just need a shot of adrenaline? Either way, don't wait too long because your half-life isn't getting any longer.