I started FCC without knowing much programing and now I’m in the middle of the React course–so, some progress. I’m making my way through it, but if feels like lots of things are taking me longer then they should.
Roughly how quickly would a typical employer expect a beginning jr. developer (or other entry position) to use the skills involved in the JavaScript challenges? Should I not even think about applying until I can just read the question and then BOOM write the code right away?
It took my boyfriend four years to learn javascript before he got hired, and even then he said it was luck.
It depends on the challenge and it depends on the position. Don’t worry too much about some imaginary finish line - just keep learning and getting better.
Should you be able to just see the problem and just write the code? There are some problems at work where I just sit down and instantly start coding because I know what needs to be done and I know the best way to do it and I can see it all in my head. Then there are other problems where I need to stop and think about it, maybe whiteboard it, maybe run some ideas past some other devs, do some research, etc. When you’re starting out, a lot more challenges are going to fit into the latter category, and that’s OK. That’s a good thing to learn to think like that because when you’re a professional dev, that is where the true challenges will be and that is where you can shine.
So just keep building and learning.
This really depends on the problem, and expectations. I would say there is always a “limit” to how slow you should be going at a given time if your getting paid to do it, but what that limit is, really is up in the air. If your just learning something, then speed isn’t that important, “learning fast” is not as important as just learning.
Not all “Jr Developer” jobs are all equal in terms of expectations.
You can get a job with minimal programming experience for a company willing to train you, these aren’t as common as a company needs to justify hiring someone and investing in their training.
Alternatively you can find those “Jr Developer” job postings asking for decades of experience in a number of technologies. These are more “we will pay you jr developer rates, but ask for non-jr developer skills”.
This might actually never happen unless you run into the same problem so many times it becomes muscle memory. Such problems are usually on the small scale, and you can still have to debug those. For example, you might know how to console.log by now, but you can still typo it and get errors later and have to debug your debug 
Finally, when your learning don’t worry so much about the time you spend, focus on how much your actually learning. You could spend 60 minutes learning 1 tiny thing, or 6 minutes, what matters is you learned it and should be able to leverage that later.
Good luck, keep learning!