The answer to that question is never. Horses don't respond to whipping besides fearing the person that whips them. Horses have good memories and they remember people and objects that have hurt them long after the pain has passed.

Many trainers unfortunately use the belief that whipping a horse will make him or her respond better and respect the trainer, however what is being created in reality is a relationship based on fear and the horse may obey commands to be avoid being whipped but will never truly enjoy spending time with a human companion that hurts them.

A better way to train your horse is by gentle reinforcement that you are the dominant one in the relationship (this will be tested now and then by your horse), and that you will not harm your horse. Instead of whipping, practice the things you want to teach your horse. If you want to teach him good ground manners, you need to reinforce what you expect every day and praise and reward him when he obeys and give him a stern, "No" when he doesn't listen or perform what you expect.

Horses have an innate need to please their human companions when there is a bond established and your horse will work hard to show you just how smart he is. Always be lavish with your praise and your horse will seek out ways to get more and more. This will in turn give you a well trained horse that will make a wonderful and well trained companion that was trained through love and kindness, not whipping and fear.