Use gloves to work a small amount directly into the root ends at the crest. Twice a week is plenty, and we only washed it out if it got gunky. Texas is like a mile from the sun, so we (a few fellow boarders did a test run) only applied at night, hoping to prevent sunburn since it's oil based. Results really depend on the individual horse, IME. Some had noticeable growth within a week, some didn't show any change.
I strongly suggest a spot-test first, however. My sensitive-skinned horse welted up shortly after applying it to a one-inch test section. I would have felt horrible if I'd applied it to his entire mane! My guy's mane is doing well with Healthy Haircare Moisturizer applied to the roots/crest every other day, instead. He definitely smells prettier than the MTG horses, too, lol.