When one uses the the form in bursts, it multiplies the base power level by x35. When the form is mastered, the base power level is multiplies x40. This is noted by Shadow in Episode 35, before he and the heroes leave the Death Egg as the RRBs and Utonium engage in the final battle.
Glimpses of the FSSJ transformation have been going about throughout the series before Episode 35. It is implied Jerome was capable of using this form in Ultima X and that Genesis may have used it to defeat him. The burst itself was more than enough for him to defeat the Mighty Tyrant.
In Episode 23, Brick implies that JaLeah could have used the FSSJ burst if not for the Chaos Emeralds near her. In Episode 25, the RowdyRuff Boys transform into semi-FSSJs, only gaining blank pupils and yellowish-white auras when doing battle with Krunckles, Zails and Redonic. Though the semi-burst only boosted their power 30-fold, they were able to defeat the incomplete Clones. Genesis almost uses this form in the beginning of Episode 26, when hearing of JaLeah's "death." Though, due to his lack of power, he only gained a yellowish-white aura.
Finally, the mastered form is used in Episode 35 when the RowdyRuff Boys all find their ultimate reasons to save the world, each personal. The Saiyan Shadows transform into the burst forms in Episode 36, and in the Text-Based special, "Race Against Time," and "Lexy's Birthday."