Chemists and Nuclear Physicists talk about the half-life of radioactive compounds. It's basically the time it takes for that compound to naturally decay to a point where only half as much of it is left. Argon has a half-life of 1.8 hours. Uranium-238 takes 4.5 billion years to decay by half.

What's the half-life of things less tangible?

Thank-you notes: if your note has full impact 2-days after the thanks is owed, when is it only half as valuable? Would you still bother sending the note 2-months later, or has the value decayed so much it's not worth it anymore?

Complaints_: When you encounter a wrong, an injustice, for how long does it help to complain about it? Certainly in the moment it may have a real impact, but does anyone want to hear you complain a week later?

An inspiration: that moment of clarity, where the idea is almost palpable, and you are inspired to act upon it. How quickly does that inspiration fade? When does the brilliant idea lose its luster, and when later do you forget about it entirely?

You wouldn't let your precious vial of Argon go to waste. So you'd best not miss the opportunity with the intangibles either!