We all have ideas. Most of them not worthy enough to be dedicated time and resources for being pursued. But occasionally we germinate an idea that seems worthy of pursual. But we do not choose to actively pursue all such ideas. But some we do. 

Every idea has a corresponding half life. The promise and the opportunity on offer should the idea be pursued falls in half after every such interval. This is true irrespective of the stage that pursual is in. 

Simple math is enough to tell us whether an idea is worthy of being pursued. The point when the decaying opportunity on offer upon successful implementation of the idea goes below the threshold that makes it worthy (this is usually the cost involved in continuing implementation), is when the idea ought to be killed.

This means we ought to constantly evaluate whether it makes sense to bring the idea to life until it is actually brought to life. 

But math doesn't easily win against emotion. Otherwise, none of the casinos would be profitable. It is the same with ideas. 

Ideas don't get killed often enough once they get past the initial filtering phase. Killing ideas in time will relieve pain and suffering. Many a time.