I’ve been asked before "how often does a horse need training?" This, however, is a deceptive question. Horses, like people, are individuals. Some horses may start out their lives with 30 days training and never need additional training to continue as safe trail or pleasure horses. Other horses may spend their entire life in training and still not be considered "finished" by their rider. This has to do not only with the horses personality, but even more with the type of rider on the horse.
For example, an honest, well started horse matched with a beginner rider who has only basic riding ability may never need additional training, but another horse with just the same amount of basic training could be paired with the same rider and need tune ups every few months. This has as much to do with the rider as the horse. A novice or beginner rider may not be able to enforce the cues they give, and may lack the skill to keep a horses training current. Depending on the horse’s personality, the horse may take advantage of them, and thus appear "untrained". This type of rider may need to send their horse offer training constantly, but if this rider took riding lessons themselves to improve their riding skills and ability to do minor training on their own horse they would not need to send their horse out for training very often, if at all.
How often a horse needs training also depends upon the way the horse is being used. For example, a horse whose only job is following the horse in front of him down a trail may never need training. On the other hand, a horse who is use for equestrian sports, such as roping, jumping, dressage, or barrel racing may need to go to a trainer fairly often to learn the skills to compete at a higher level.
Many experienced riders are able to train their own horse even if they are not horse trainers. It takes a varying level of skill to train a horse. A competent beginner/intermediate rider can maintain the training on the average horse, and an experienced rider may be able to take their horse to the next level without having to send their horse out for training often at all.
If you are horse owner or potential horse owner wondering how often you might have to send a horse out for training, consider lessons for yourself instead of training for your horse. Riding lessons, especially with a trainer who will teach you how to deal with problems with your horse constructively, are a much better investment than frequent professional training on your horse. If you learn along with your horse, not only does your horse receive the training it needs but you will become a better rider so your horse will not take advantage of you, and in the future you can help train your own horses.