I've been teaching GRE prep for about a year now. The answer to this question depends a lot on how many hours a week you're willing to put in, and by how much you're looking to improve. Generally speaking, I work with most of my students for 8 weeks at about 3 hours a week. Apart from these lessons, I expect them to put in about an hour a week per subject (reading, writing, math, and vocab). This is a lot of work, but my students generally start between 145-150 per subject with 4's on the writing, and end up at 160+ with at least a 5 on the writing section.
No matter what, the first thing you should do is download the GRE PowerPrep software (http://www.ets.org/gre/revised_general/prepare/powerprep2/download) and take a pretest. Test prep is primarily about practicing the questions (and, in this case, learning tons of vocab!). The more you practice, the better you'll do! You should plan about 2-3 months to prep for the test, but make sure you'll really have enough time to devote to it.
If you're looking to apply to a graduate program this spring, you'll probably only have enough time to do some crash prep work in one section and study vocab. If you're planning on applying next fall, you'll have lots of time, so you can really focus on in-depth studying.
Feel free to contact me if you have any extra questions! Good luck!