Brooke- I am a really fast washer...I wet the horse down really good, then lather up with the red stuff from rear to head on one side in less than 5 minutes. Then I count to about 100 and then wash it off. Trick is not to let the suds dry - it's just bad for the skin in general. Then I do the other side....so the answer to how long would you leave it on would be no longer than 5 minutes.
Corn oil is my secret - 30 days will make your horse shine. 1 cup in the food a day and they will glisten like they just got sprayed with showsheen. For regular baths, I just use Dawn liquid soap but no more than once every two weeks with it. Remember it is a degreaser and will thoroughly clean the skin, oils and everything (I use the antibacterial Dawn soap but not on their head) so I've had really good luck over the years with the above regimine - just the corn oil and Dawn - no flaky manes, no itchy tails - no crusty skin.