More than a shelf life, delights have a half-life (and it’s becoming increasingly shorter). The key is that the value of the half-life is tied to the frequency of the delight.
As you state further down, the best times to inject delights are during influential, one-time events.
Quick, funny story
For a user permissions feature, we were going to include a little delight. It would show a little wizard GIF when you granted someone all the permissions. It was cute. Luckily, we decided not to add it out of the gate. Turns out (based on usage), people were granting all permissions a lot. So we decided to scrap the delight (much to the disappointment of the designer).