IMO
Technique: If proven by extensive testing that it's unbeatable and is within human capacity - Immediately
If the technique is associated with a specific character and cannot be discretely banned, meaning it's hard to tell when the technique is being used, then the character should be banned, but with a month or so of warning.
If the technique isn't character-specific, it's time to suck up and admit the game is broken. Change the way the game is played or simply play another game.
If the technique is not unbeatable but simply very good - most likely never. However, if it's character specific, it can contribute to the character being ban worthy, depending on the rest of the character's traits. But that needs more time to be evaluated.
Character: As soon as there are warning signs that a character's worst matchup is itself, there should be a season (or year) worth of tournaments and metagame development to confirm whether or not that still remains true. The reason I say this is, that character becomes the focal part of the metagame. If a year passes and nobody has found a way to go even with it, even though it's the main objective, then the character is bannable.
Stages: Way too complicated and dependent on the tournament populace's view on competition.
edit: prefaced with IMO