I think this will vary widely by both the specific job in question and the learner. What takes someone a month might take someone else 4 months, and that discrepancy doesn't always necessarily mean the faster learner will be the more competent developer in the long run.
The best way to get a sense of where you are is to try it. Start applying and go through the process, aware you will likely not succeed at the beginning. You'll learn a lot about your current strengths and weaknesses based on where you excel and struggle in an interview, and it will give you a clear path for what you need to be focusing on. Its' incredibly difficult to make this assessment accurately yourself without context.
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