I don't know exactly how long it will take, in terms of hours worked, but in my opinion it is possible to prepare for N5 between now and December.
However there is one important point. In order to pass the JLPT you have to specifically prepare for the JLPT. I know people whose Japanese is much better than mine, who thought they could pass the exams on the basis of their general level of competence, and failed. On the other hand, I successfully prepared for the N3, and passed it with a good score, despite the fact that my Japanese is not really up to that level. I am hoping to pull off the same trick with N2 this year.
The JLPT exam is a race against the clock. It is not a test of general competence; it tests only recognition, not reproduction. It has lots of "trick" questions that you will become familiar with through practice. Preparation involves cramming, exam planning (time management) and above all practice.
Anki will help you build the kanji and vocab for N5. You should be able to find and download decks that are specifically designed for N5.
Apart from that you need a grammar textbook that is specifically designed to cover N5, and you need books of mock exam papers that include CDs.
Best of luck,