His point is that your question "how long should it take to learn" is very difficult to answer, since it implies that there is a point at which you have actually learnt everything about it. I can say that I know English, after all it is my native language, but I still have to take English class because I can't know all of it.
It should take you a couple days to completly understand the text commands. Of course, on the first day you can probably get a rotating cube on your screen, but there is a lot to learn when it comes to 3d commands.
I agree with TDK's estimate of 12 months to get to a good standard. The point that (I think) flibXOr was making is that it doesn't stop there.
