I picked up c# within a day or two with a working knowledge of Java. Some languages are easy to pick up given the right background.

I'll have to check erlang out. I only hear good things.

What's your definition of "picking up" a language?

A) Writing a "Hello, World"?

B) being able to fix a simple bug in a program?

C) Writing a small program that solves some domain problem

D) Writing a program that solves a "real world" problem, making use of the programming languages' strengths and conforming to standards (something that other people would let you commit into a repository unchallenged :) )

I'm certain A) takes only a few hours, and B) can be done in a day.

As Norvig says, you can do C) if you write in the new language like you would in a language you already know (not much of a problem if we're talking C# vs Java).

But D) IMO takes weeks/months.

This roughly corresponds to my experience with Erlang; keeping in mind that "D" in Erlang relies heavily on its OTP ecosystem, which is fairly complex and different from most other languages.

Ignoring OTP, my ability to use the language proper was pretty much complete within a week, owing to its simplicity and my prior background in functional languages.

This is in contrast to C, which I have been using for around 15 years now, yet am still learning nuances of the language. (Things like: which integer operations are undefined on negative numbers; how integer promotion works with shift operators; how "restrict" interacts with scope.)

C is almost a fractal of nuance that does take years of experience to comprehend; Erlang has no nuance. (The closest thing to nuance I can think of is the relationship between integers and floats; and even then the takeaway is "it just works; don't worry about it". The only language in which I've seen the number hierarchy handled more cleanly is Racket.)

Erlang has a few syntactic conventions that will probably seem familiar only if you've used Prolog. But, like any other language syntax, you learn it pretty quickly and within itself it starts to makes sense.

This was actually the biggest syntactic stumbling block for me, but because I know Prolog. It's definitely not Prolog semantically, and differs syntactically as well (e.g. clauses are separated by "." in Prolog but ";" in Erlang); to this day I sometimes find myself writing Prolog in Erlang.