I didn't have c# exp and it took me a month to feel "productive comfort", and less than 2 months until i felt "native comfort". I've been using it for about 2 years now.

That said I:

• had zero C# experience and it was the C# interop that caused me the most pain. Its easy now but I didn't grok how to properly pass arguments to overloaded C# methods (i kept messing up parenthesis, commas, and didn't pass argument names to tell f# the right function signature)

• have been coding for 8 years

• have functional lang experience with elixir and elm.

Now f# is my primary backend language.