Done once the image quality deteriorates.(posted 8660 days ago)
This technique is usually used for intensification, and as such it works very well indeed. The only time I used it was with some 120 negatives that turned out very weak after normal development. Bleaching and re-developing gave normal to high contrast negs but the grain was really excessive, even with MF.
Not recommended, except as a last resort, or for grain effects.