Generally, I recommend 20 minutes, 30 minutes is ideal if there are no significant votes prior to 30 minutes, and you know the post will do well when it finally gets discovered. It's virtually impossible to predict, that's why I recommend 20 minutes. You will give 1/3rd to the author but you generally will make more in the end.
Smaller posts generally 30 minutes is a good rule of thumb as they take longer to be discovered but run the risk of never being discovered.