The taller the horse, the longer it takes. Fully grown, you're looking at 7 years. Back is last to mature.
Unfortunately we're having the throw away mentality with horses too.
It used to be start them slowly in late fall, after pasture season, in their 3-year old year, ride them lightly during winter, and throw them out on pasture the next summer to mature. Then, after pasture season, more serious work would begin.
Nowadays people want a 3 year old which won the Olympics and has 15 foals on the ground which are world champions........sad.
A very thorough full PPE is an absolute must for this horse. And not only physically...mentally also. I've seen plenty of horses purchased at the 3 year old riding horse auctions for 6 digit prices which need to go on pasture for at least a year, because they're completely out of their mind and unrideable.