What's your experience in programming? Do you have any background in C or C++ by chance? If you have a decent programming background you should be able to do games in a matter of one month (minus development & testing). If you are picking up Java as a first language then you might be in for quite a ride before you can really break into game development. After a hard core month of 12hr days studying 38 different Java books mixed in with developing via trial and error through the command line, I had a fair grasp of Java enough to write simple games. As of late, I've done several card games, a few board games like sorry and monopoly, but nothing I could release to the public considering how crued they are. Once you've been programming for over two or three years (in any language) you'll have a grasp for what you need to do.
If you want to program in Java for cellphone games, you should make sure to focus heavily on the latest J2ME. One component you will want to make is your own GUI elements rather than utilizing Swing or AWT. It's a bit more polished when you use your own style.
Based on your programming background I would be happy to recommend to you a few books as well as example java code. Let me know when you get the chance. I'm an SCJP so I can help answer whatever questions you may have. Though, there are others around here who have more knowledge in Java as well as programming games.
:pint: