Keep the water running until no more soap runs out. After rinsing, I use the sweat scraper on my horse to remove any excess water, followed by a good toweling. Finally, I walk him until he's dry. If he's not, my horse will immediately roll in the dustiest spot he can find, and it'll be back to the drawing board. If it's chilly or windy, you might want to toss on a cooler that will pull moisture away from the skin, and keep him a little warmer.
Horses generally dislike having their heads washed, so the task takes a little more planning.
Don't use the same amount of shampoo you used, proportionally, for the body, but use a separate bucket with minimal soap. That translates to less rinse time. Using a small sponge, I gently wet my horse's head, massage in the shampoo, then rinse. Tails and manes are tricky, too. Fold the tail in half, approach from the side with a bucket filled with warm soapy water, and dunk the tail in.