This is another difficult question, because the answer is different for everyone.
Malcolm Gladwell writes that it takes, on average, 10,000 hours of practice and study for anyone to become truly proficient at something.
For myself, I just added up on a calculator all the time I spent studying/practicing animation in school (including stuff like figure drawing), time with my mentors, and animating on my own. I added all of that up, and it turns out that my animation education time before landing my dream job at ILM was 18,400 hours.
That might sound daunting, and it clearly took me more than the requisite 10,000 hours, but really we're just talking about 5 years of focused studying in order to have a reel that got me into ILM. I have no idea if that's average, fast, or slow, but we're all going to have different speeds as we study and absorb this animation stuff.
I would guess that the number of hours "necessary" to excel in animation would be radically different from one person to the next. I think the actual number of hours is irrelevant -- what matters is how much you can focus on animation.
For me, especially during the 4 years where I was focusing the most on learning this stuff, animation was my life. If I was awake, I was animating. That literally isn't really much of an exaggeration. I wasn't going out much with friends, I didn't have a girlfriend, I was missing movies I had been excited to see, I didn't get to play the video games I wanted to play, etc. Animation was my life.
Maybe that isn't always the healthiest way to approach something, but it's probably the fastest, and certainly works.
I know not everyone has the option of focusing their whole day on animation - you have families or non-animation jobs and other responsibilities. Well, my advice then is to just focus as much time as you can. If all you can do is 5 hours every night after your kids go to bed, then start spending those 5 hours animating or studying animation.
5 hours a night is 1,780 hours per year. If you manage to squeeze in an extra 10 hours each weekend, then you're talking about 2,300 hours per year, and suddenly that 10,000 hours doesn't seem so far off!
I wish animation was something you could learn in a few months, but it just isn't. It's an endlessly complex art that takes a lifetime to master, and that first 10,000 hours will only get you to the tip of the iceberg. Luckily for us, the rest of that iceberg is a ton of fun to spend that next 100,000 hours exploring!