dandruff in horses is caused by the skin being dry and flaking off so make sure you use a very hydrating shampoo and condition to moisturise the skin. Can also massage a baby oil in before a shampoo to help pull out the flakes while hydrating (I'm a hairdresser)
Use Kevin Bacon's Lucy Diamonds shampoo :)
I curry the horse for around a half hour maybe more maybe less depending on the horse. Or you can give a bath and try to srub it off. MTG works great also after you scrub it!!
In horses? Mane&Tail shampoo / exfoliating brush. In humans? Head & Shoulders 2in1 shampoo