Nobody can guarantee that your horse's mane will grow back the way you want. The same conditions that caused it to bleach will still be there, and you'll have to keep up on her grooming if you don't want it to tangle and form knots.

How do I get my horse's mane to grow back?

Nutrition – Feed and Supplements

If your horse is lacking nutrients, it may due to a mineral or vitamin deficiency it may delay regrow and reduce the condition of the mane. To encourage re-growth, try using some supplements jam-packed with goodness! Another thing to check is the quality of hay/haylage they are eating.

How long does it take for a horse's mane to grow back?

Manes? Anecdotally, it appears that a horse's mane will grow anywhere from half an inch to 1.5 inches in a month, depending on the breed of the horse and other factors. Often, heavy breeds like cobs and draft horses will have quicker mane and tail growth, and the hair will be thicker.