Having thrown knives myself as a (casual) hobby when I was younger I can tell you that it doesn't take that long to gain both power and accuracy with well balanced knives. Give it a few months of everyday practice and you can hit a one inch target at a good few meters distance with a 90% success rate. Well balanced in this context means that the handle and blade weigh the same, so that the knife balances where the two meet.
The really tricky part though is judging the distance. How many times is the knife going to go end over end in the travel from hand to target? If holding the blade when throwing you need (N + 0.5) rotations. If holding the handle just N. But the number of rotations depends on the knife, both the weight and the length, and that just takes practice to learn the feel of that particular knife. To give some real-word examples, a small 4 inch (blade + handle) throwing knife might do 3.5 rotations over a ~3 meter distance, but a 12 inch knife may only do 1.5
In fact there is only about a 45 degree arc during the rotation where the knife will actually penetrate the target at all, and the closer to perpendicular the better. Anything outside of this angle range and you are hitting either handle first or flat. But getting that right every time is very hard if you are not throwing a known knife from a known distance. When we used to practice we threw from marks on the floor that we had worked out through trial and error (lots of error!) and always used well balanced knives.
So perhaps the real mark of an expert is in how quickly they can judge the characteristics of a new knife, and also how well they can spot a distance. Only a genius could pick up a knife for the first time and get a penetrating hit from an arbitrary distance though.
In the real world, the utility of throwing knives is therefore slightly questionable in my opinion, as the distance to your opponent would be extremely hard to judge in a real fight. As a distraction technique I can see it having some use, or perhaps even as a ranged stealth attack with time to prepare a throw, but I can't see it being favoured as a primary means of attack where there are better options available.