At what level have people become comfortable with a language?
I'm curious to know if anyone has become fluent or fluent enough to have conversations with people in the language they're learning. I know that without real world practice, it isn't possible to become fluent but is it possible to get pretty comfortable in a social setting after mastering a language on Duolingo?
Depends on what language you're learning. One of my favorite ways to practice French was to change the language of my favorite films, the ones I knew all the lines to, from English to French. I also listened to French news, and I listened to audio books read in French. These methods are practical in other languages as well, and it's always a good idea to listen even if you don't understand everything. Reading is also a plus as it gets you used to being able to read at the speed with which you read English.
Novels may not help you become fluent. They are written in a language that is very different from the informal spoken French. However, you said 'pretty elementary novels'. They would have to be in a very simple and almost informal style, then.
'Le Petit Prince' is not as simple as it looks. It is poetic and original. Not the best book to learn from.
I think you are fully fluent when you have lived in the place for at least a year or two and you made connections with the people there. I think you feel comfortable when you start speaking without a doubt to your friends and/or if you learned a bit of the slang (or the "less formal way or saying things":
"I have got to go get bread." = "I gotta get bread."
I like that girl because she is so pretty = J'aime bien cette fille parce que elle est jolie = Je "kiffe" la "meuf" pars'qu'elle est "chaude".
Al languages have that!
Depends what language you're coming from and what language you're trying to learn but using only duolingo alone it'll be difficult to have a reasonably complex discussion with anybody. In particular I find that Duolinguo tends to be pretty light on the grammar.
There are several paths towards fluency depending on how you like to study and what your objectives are. If you're mostly interested in the spoken language then working on oral comprehension is capital.
You could try looking for the french version of movies you already know, this way you can focus on the language and you won't get frustrated if you don't understand everything since you already know the story. I think cartoons work best for that. Try to find the french version of your favourite Disney movie or japanese animé for instance. You'll probably want french subtitles at first though, it'll make it easier to decode what the people are saying.