Bleached hair cannot "turn back" to its natural color, including black, on its own. Once the melanin pigments are removed from the hair shaft through the bleaching process, they do not regenerate naturally. To return bleached hair to black, you would need to dye it with a black hair color.

The time it takes to dye bleached hair black depends on the dyeing process itself, rather than any natural regrowth. Here's what you can expect:

1. Dye Application: If you decide to dye your bleached hair black, the actual application process typically takes about 30 minutes to an hour, including the development time specified on the hair dye package.
2. Choosing the Right Product: Select a hair dye that is suitable for previously bleached or lightened hair. You might need to use a color restorer or a permanent hair dye specifically designed for covering lighter hair with a darker shade.
3. Prep Work: Before dyeing, ensure your hair is in good condition to receive the color. Deep condition and moisturize your hair to help it better absorb the dye.
4. Aftercare: Post-coloring, maintaining the color can be challenging, especially since bleached hair may fade color faster. Regular touch-ups and color-safe hair care products can help prolong the life of the black dye.

Remember, going from a very light color to a very dark color can be a dramatic change and may require multiple dye applications to achieve the desired depth of color, especially if there are still very light or resistant areas in the hair. It's always a good idea to seek professional advice from a hairstylist for such a drastic transformation to ensure the best results and minimal damage to your hair.