Usage in CanonEdit
The Saiyan Fighters have used this technique many times during the series. At the beginning of the series, the Kaioken ability was only used up to Kaioken x2-4. But in episode 6, "Shadoo of the Past", the Evil Trio were beginning to overpower the heroes. As a result, they were forced to push it up to Kaio-ken x6.
But as time went on, the fighters developed an endurance to the Kaioken (Seeing as it takes much energy and strength to subtain it), allowing them to go up to Kaioken x10. This transformation is used throughout the entire duration of the Mecha Saga and for most of the Redux Saga. The transformation is mainly used at x10, x12 or x20 for the battles in these arcs. However, in the fight against the PowerPunk Girls in Episode 38, it has been implied that they have discontinued use of the Kaio-ken, seeing how vastly superior the False Super Saiyan is to the transformation.
The Kaioken levels have been stated to go from 1-20. Though the effects go x1.5-x20. 20 being the highest achived. If the fighter can withstand the energy level of Kaioken, no strain is felt, unless they overuse the Kaioken. An example being in Episode 12, when the RowdyRuff Boys use the Kaioken x20 to thwart Bowser's Dark from, but are thrashed by their incomplete use of the form.