Well, as you say, this all depends upon the individual, how familiar he is with the language and the specific exam topics and so on. So, I'm just gonna tell you what my learning experience was when I studied for the SCJP back in 2008. It's not the exact same exam, but the idea is pretty much the same.
First of, here are the exam topics :
-Working With Java Data Types
-Creating and Using Arrays
-Using Loop Constructs
-Working with Methods and Encapsulation
-Working with Inheritance
(more details on each in the link, or if that doesn't work look it up on the Oracle site).
What I first did is get 2 books on this specific exam. It's important to have 2 in order to have sort of a second opinion on each of the topics, because sometimes one opinion might not be clear, basically they may give the idea that some parts of a given topic are more important than others, and this could lead you to under or over-prepare on some things. If you can't find two different books on the exam, try replacing one of them with just a book on the basics of Java, there are tons of very good such books out there.
Now, once I had the 2 books, saw how big they were and also looked at the list of topics, I just approximated a study time for each topic, trying to lean on the generous side where I wasn't too sure on the estimate. For me, all this took about 4-5 months of studies, but I was working at the time, so most of my studies were crammed in weekends and late nights.
However, after doing all this, what I found more important is to make sure that I understood what I just read. This basically means doing A LOT of questions and exercises where you just write small snippets of code. You should go over each topic and do this. I can't stress how important this is. The exam-specific books come with a good amount of "simulated" questions, do these several times so that you'll get familiar with the stile of the exam questions. Also write code snippets. Don't use an IDE, if possible don't even use a code highlighter (like Notepad++), just write small snippets of code in VI or Notepad, compile and run them in the console and see what you're missing.
For me this took about 7-8 full days.
Finally, once you do all this, go back to the list of topics, and honestly highlight those you're feeling uncertain about. For those, re-read the corresponding chapters in one of the books, and redo the questions and exercises.
I did this as well, took me another 3-5 days.
Hope this helps in guiding you to a decent approximation of your study time for this. Good luck!