For a young horses' joints to fully fuse, it takes 3-5 years. This is why horse racing is very dangerous because the horses are not fully developed. Do not jump/race/work hard with immature horses!
Really it takes as long as it does for you to fill all the bars... you could do alot of traing in one day or a little each day over an amount of time... just as long as you are putting the well being...