I think how long you study for the GRE depends on how well you think you will do and how much time you actually have. Knowing that you work and take classes, I would prioritize the verbal and writing sections as those are generally weighted more by admissions committees. I found that most schools say they require about 50th percentile for each score but would prefer higher for the verbal and writing sections. If math is a struggle for you, as it was for me, I would recommend doing a somewhat quick refresher of basic concepts (I needed algebra especially) but then move onto the other sections. The GRE is an adaptive test, so sections may get more difficult or easier depending on how you did in the previous session for that subject. My first section of math went pretty horribly but the second section (which had the easier questions) included the more basic concepts I studied so I was still able to pull off about a 50% percentile.
For verbal and writing, a lot of your studying depends on how much time you are really willing to dedicate to studying. I mistakenly viewed the GRE as somewhat of an afterthought and didn't begin looking at materials until two weeks before my test. Because of this, there was no way I was going to memorize a ton of new vocabulary words. I focused my attention instead on analyzing the different questions they ask on the sections and the best ways to tackle them. I actually write standardized test questions for my current job (excited for a change!) so I can tell you that there are absolutely typical questions these test writers use and with some reasoning you can figure out the answers without even knowing the real definition of the word. I used the new kaplan book and it's online sample questions for this and scored a 161. I did not know the meanings of all the words on the test, nor did I feel like I had complete understanding of all the critical reading passages, but I was able to narrow a lot of my answers down with test strategies that I felt like Kaplan was helpful with. So, if you are in a bind for time, studying hundreds of vocabulary words won't be helpful to you. You really need several months of using those words for them to actually become an integral part of your vocabulary. I think you can, however, master some test strategies pretty quickly that will at least better your odds of a good score.
For writing, I also found the Kaplan book helpful. The essays for the GRE always follow the same two formats: developing an argument yourself and analyzing someone else's argument. If you know how to then form your argument and know the fallacies other people use in theirs, it doesn't matter the topic you are writing on. Like lcs2074, I also looked up 5 and 6 level essays as examples and found myself referring to them while I was writing my own. These examples can be really helpful and can give you clues on how to structure your essays. For my first essay, I remember following the structure of one example I read pretty closely. The way people form good arguments tends to stay the same. So as long as you can back up what you are saying with evidence and examples and you are able to do that cohesively, you will do well.
Overall, if you have the time to really dedicate to studying for the GRE, you should take the time and really put your full effort into it. I really do think it's a good way to distinguish yourself from other applicants, at least academically, and is especially helpful to counteract a low GPA. However, if you are unable to dedicate hours and hours to studying there are some quick things you can do strategically to better your score a bit. Good luck to you!!
