This was one of my example questions but I now am actually interested in the answer. I am an amateur conlanger; I have one pretty complete language and a couple in the process. But I can't construct a sentence in any of my languages without referencing the lexicon and grammar. How common is it for the creator of a conlang to be fluent in that conlang? (especially for languages that are not meant to be spoken like all of Tolkien's, where there is not a very significant fluent community).
From personal experience (active member in a large server of avid conlangers) very few people ever learn to speak their own conlangs. Language learning is a very time-intensive progress and there is not much gained from learning a language no one else speaks. The occasional exception tends to be found in group projects, in which sometimes at least to a part “creating by using” is used as a technique, which naturally leads to at least rudimentary command of the language. Some languages have also been developed exclusively by using them, such as Viossa, but this is an exception.
You may also be interested in this talk at LCC6, held by Jim Hopkins in his conlang (with an interpreter, of course). Summary of the talk:
In this presentation Jim Hopkins, creator of Itlani, will share his tips and techniques for recording, learning, and living the Language of your Soul. He will discuss the need for finding and ensuring stability in your language’s vocabulary and structure to facilitate easier learning, and how to immerse yourself in your language for mental and emotional engagement, even though you aren’t living full time among native speakers. The presentation will be done entirely in the Itlani language, with interpretation into English by Tony Harris.
It's probably a lot more common for a person to be fluent in his own conlang when dealing with auxiliary languages. Esperanto, Ido, Interlingua, etc. all have large communities of speakers (relative to most conlangs), and I assume their creators also spoke them.
This is likely because the languages were designed for communication, and because the vocabulary is usually designed to be easily recognizable to as many people as possible, which makes learning it easier than learning an a priori vocabulary and grammar.
An example of a relatively recent auxlang is Lingua Franca Nova, created in 1998, which already has a lot of speakers. By contrast, a systematic "engelang" (engineered language) like Ithkuil would be extremely hard for anyone to master, and its creator is not fluent in it.
Ignoring that fluency means a wide range of things (reading, writing, conversing, conversing at a college educated level, etc)--
This is really rare. Outside of Esperanto, which as L1 speakers, the number of people who get to the lower levels of fluency is in the low two digits. Klingon, Na'vi, toki pona have up to tens of people who have various levels of fluency. Getting solid data is also really hard, the numbers you see in wikipedia are often wild ass guesses.
As a thought experiment, you can look at how many languages are even potentially speakable, ie. is the specification complete to say most or all possible sentences? Is there a community? Before the internet, this was usually an insurmountable problem, finding anyone who'd want to learn your language. Are there any "poison pills" in the language put there by the designer specifically to discourage humans from trying to use it? (Pedophilia in the conculture and absurdly difficult pronunciations come to mind)
John Henry did a study of this, targeting more recent personal conlangs.