If you have familiarity with other language-based OOP, like PHP and C++. Learning Java will take nothing more than a morning or an afternoon. But if you are totally newbie for programming in general, then, this will take more time; a month or maybe, more.
Where can you learn it, then?
Well there are lots of tutorials on the Internet and ebooks -- to get you started, just google it.
Java has a lot of use in Android development these days, but people are fearing full dedication for Java after it had been purchased for Sun, cuz no one knows what can happen in the future with Java and MySQL. Reason for that is that C language became the 1st language used world-wide, place that had been occupied for Java in a near past.
Then make your own choice about it.