That's a difficult question because there's no time span going from "I don't know anything" to "I know the language". I have no idea how long it took me to learn FoxPro. I finished my first FoxPro program for my employer 4 hours after I got a copy of FoxPro. That doesn't mean I mastered FoxPro in 4 hours, but since then I only kept adding what I needed. There was never a point when I said, now I have learned FoxPro.

The same is with .NET or Java, or any other language... It never takes very long to get starting, development is a bit slow the first time, but never that slow, and in the end the result is a program that works but that you would do differently if you started now. A week is probably enough to get started with the new environment, learn the IDE, learn what tools are available at least to the degree that you can build code into an application and run it. For the remainder I would reduce the productivity to something like 50-75%, depending on how easily you can get around the "in VFP I would do it this way". And depending on how easily you can give up on the idea that the first Java application must be a perfect one.

In the end it comes down to two big blocks: How do I build and structure an application in this language? And a gazillion questions of small things like how to manipulate strings, how to add a day to a date variable, etc. For the first one it helps to participate in training course or get a book, for the second one the internet will usually provide dozens of samples for every question.