His improvement seems to be fairly rapid. I guess if this is your sole pursuit, not that surprising. But nevertheless makes I'm rooting for him. I think it would be testament to the theory.
Overtraining is what happens when an athlete's training frequency is greater than their body's ability to recover. That is, by the time the next training sessions comes around, they haven't yet recovered from the last one. This can harm overall performance for obvious reasons.
That may lead to a plateau, but I think plateau's happen more often when the athlete ceases to learn from training sessions. And that happens when they're just doing the same thing, over and over.
All practiced activities show diminishing returns. If you practice 5 days a week (assuming your body can handle it), the benefit of practicing 6 days a week (assuming your body can handle it) is not going to be the same as going from 1 to 2 days. But if your body can handle it, you probably will still improve.
Have you ever seen personal bests charts? Save specific cases of breaktrhoughs, it plots very logarithmic for everybody. That is diminishing returns, more or less.
So if he fails, it says nothing about the theory. Should he succeed, that's a whole another story and would be pretty exciting.
How long did this take? About five years.
He bitches now and then that he's the unpaid master class of the Hollywood TV industry. Not everyone (me, for instance) who's done time at Spümcø is working in the animation industry. But everyone who spent much time doing John's directed practice can draw rings around most people with their eyes closed.
The hard part, IMHO, was putting in the time to practice and keep practicing. Even on a shitty day. This guy seems to have that down; he's been able to structure his life so that he doesn't have much to distract him from golf. If he keeps at it and doesn't get run over by a truck, I'm pretty sure he will either become a pro or burn out.
But the great animators or coders are those who can produce great value under normal working conditions, which are much more similar to practice conditions than the competitive environment in sports.
I can think of one area in business, which is more like the competitive environment in sports: negotiating. The pressure caused by these situations shows easily in your non-verbal communication, and if you appear lacking confidence, it can undermine all your further efforts. The pressure makes some people to crack, but few people can gain extra motivation from it and are able to use it to their advantage. The sports experts often say this is what separates the very best athletes from the rest.