I've been reading Pragmatic Thinking and Learning by Andy Hunt (who co-wrote the The Pragmatic Programmer) lately. The book brings up idea of second-order incompetence, which means that a novice at a particular skill is so unskilled that they don't even realize how unskilled they really are. This leads beginners to greatly overestimate their own ability. In many cases, it can lead a beginner to be more confident in their own skill than an expert.
The second-order incompetence theory applies to any skill in general, but I've noticed that it applies extremely well to programmers and their particular set of skills.
I've heard some programmers say that it takes anywhere from three days to three months to learn a new programming language. I personally feel that this is an extremely low estimate (I've been learning C++ for several years). Three days is barely enough time to learn the syntax of a new language. Three months is likely enough time to learn the syntax and a few common libraries of a language, but it isn't really enough time to get you beyond the "Advanced Beginner" skill level. It's probably just enough time to give you the confidence you need to be extremely dangerous.
The book goes on to explain that it really takes more on the order of ten years to truly master any non-trivial skill, whether it be playing chess, playing music, or flying an airplane. Peter Norvig quotes some of the same research when he advocates the ten year time scale for programmers in his article Teach Yourself Programming in Ten Years.
These ideas are hardly new. Socrates was onto a similar idea almost 2500 years ago when he said
"True knowledge exists in knowing that you know nothing."
In programming, as in almost any worthwile pursuit, this means that you have to give yourself time to reach a skill level high enough that you know how much you don't know. If you catch yourself feeling confident in a given language after only three days (or even three months), try to realize that you've probably only seen a tiny fraction of the what there is to see. Remember that 90% of an iceberg is below the surface.