How much time does it take to get good enough at Java to make a very simple game?

Nine weeks to follow along in a computer book demonstrating Java game programming. About 2 years to turn your very own ideas into code. Reading the books makes sense as you glance back and forth between the diagrams and the text. Doing the code examples at the end of each chapter reveals the concepts that we missed while just reading.

It all depends upon how fast you learn, how much you already know, ability of using the correct code at the correct place, knowing how to optimize it, etc.

If the game is as simple as a X-and-O game, then it will not take much time: about 2-4 weeks should help you learn the basics of the awt, swing, graphics packages in Java.