It took me about 6 months to an year to learn core Java.
You can start with head first Java then read the OCPJP book both by Kathy Sierra.
These two books makes you intermediate.
Learning SQL is also required to find job !
If you have 20 hours per week, I’d say it would take you a little less than a year if your efficient with your time.
You might be able to speed this up if you jump in early and start trying to do projects, and have access to good resources. (books, courses, online classes) There’s also extra time you might need to factor in if you wanted or needed to learn technologies to apply for jobs (like Spring Boot for web apps) The “less than a year” projection is a rough estimation from my knowledge of colleges that teach programming using Java.
I’d say its more important to learn programming in general than a specific language. But your aiming to get a job sooner rather than later, learning the language you would like to use at the job is a time-saver.
Finally, I also recommend learning, or at least getting into some basic SQL technologies. Java is a back-end language, and is often used with SQL databases, so being knowledgeable in both is important if you want to hit the ground running.