If I had an experienced engineer who'd spent years writing C++ and C#, the answer would be a couple of days, or, frankly, apply without learning it and deal with it if you get the job. Because it's not Java, the language, which is the problem. It's the fundamentals, the actual skill of programming. When someone says "3-5 years of Java experience" what they're really saying is, "I need someone whose programming fundamentals are completely solid in the rough domain I'm dealing with and who's had to deal with the quirks of the JVM enough to not be brought to a halt by them."
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If I had an experienced engineer who'd spent years writing C++ and C#, the answer would be a couple of days, or, frankly, apply without learning it and deal with it if you get the job. Because it's not Java, the language, which is the problem. It's the fundamentals, the actual skill of programming. When someone says "3-5 years of Java experience" what they're really saying is, "I need someone whose programming fundamentals are completely solid in the rough domain I'm dealing with and who's had to deal with the quirks of the JVM enough to not be brought to a halt by them."