Well, simply put, the time that riders spend with their horses - and what they do with their time - determines how 'masterful' they are at horsemanship and riding.
For example, if I'm not too busy, I ride about 5 days per week for between one and four hours. Which means I ride an average of 260 to 1,040 hours per year (a big range!) That means it will have taken me 10 to 20 years to 'master' riding. Of course, what I was doing while I was sitting on the horse matters, and since I've done a lot of things while 'up there' I'm not particularly masterful at any one type of riding. However, I'd say that the time I've spent in the saddle has allowed me to become a reasonable rider.
But what about the horses? Not only do riders ride, but they also spend time grooming, tacking up, doing groundwork, lunging, and schooling their horses in other ways. So each horse that;s in a program may receive between 260 and 1,040 hours of riding hours per year, plus let's say an additional hour each day, 5 days per week, of groundwork or other schooling time. So every year, a horse in a reasonable program 'practices' for 520 to 1,300 hours.
If these assumptions are correct, then it will take a horse between 10 and 20 years to achieve 10,000 hours of practice (riding and groundwork), which would presumably make them a reasonable equine citizen. Of course this is assuming that the rider/trainer is doing a lovely job and is skilled enough to help the horse 'practice' correctly. Hmmmm.
This is what I was thinking about as I hacked my rising 7 year old down the road ponying a 14 year old, both of whom though they needed to walk like the devil was after them and spook at every little thing they thought looked a bit 'different' that day. I was thinking.... Are they afraid or are they being silly? Do they need more training time? Different training? More groundwork? Or do they just have a bit of spring fever, like me, and want to go faster and farther? I hope it's the latter. Ten years is a long time...