I would suspect that learning functional programming itself, after pretty much everyone gets started with imperative, could take as much effort as learning OO from the same imperative base (but of course a lot more people now start with something like Python and at least dabble with OO).

I.e. more than an afternoon or probably a month to get good, vs. the few days it takes to learn another language with familiar paradigms.

Not sure from personal experience because I learned functional style Lisp/Scheme really early, more than a decade before OO, which made learning Clojure a relative snap. OO in the form of C++ was harder for me, but maybe functional is just easier for my mind to wrap around. The quality of the Lisp and Scheme texts I used, especially SICP, also probably made a difference (the OO texts I used were good, but not as great, with the exception of OOSE (http://www.amazon.com/Object-Oriented-Software-Engineering-A...), but that's as much about great design of your project as well as what the project is making).