You can expect a pretty sizable jump from your cold scores. Generally you'll get big gains as you start studying, and then you'll plateau a bit. Luckily, though, quant is easier to improve 10 points in than verbal. Quant is fairly linear, in that you can learn concepts and improve your score directly. Learning how to read well is a much less straightforward proposition.
These are the official materials: Official GRE Super Power Pack. You'll want to have those books and know them inside and out. They are as good as it gets, and well worth your time.
Make sure to keep an error log, and pay close attention to the problems you are missing. Why are you getting them wrong? What concepts are you not confident with? What traps are you falling for? This will help you identify and target your weaknesses, to better focus your study time. Don't move to quickly, but keep coming back to those hard problems and concepts, even if you think you know them.
Otherwise, I'm here to help, so feel free to reach out if you can any questions along the way! Seems like you're on a good path, so keep it up.
-Magoosh Student Help
Honestly, everyone's different. Some people need more than others. My general recommendation is to start with a powerprep test to get a feel for it. Then work on the official material for awhile and see how it goes. You'll want to own those materials no matter what.
If you're doing well you might be able to get away with just that stuff. If you're struggling, you might have to look into other options.
Online programs like Magoosh will have recorded lessons for you to watch instead of reading. Then there will be practice problems with explanations and access to real support. But they're more expensive than just books.
What have you been doing so far? How are you feeling about it? Do you have the official material? Do you think you'll need more?
Best of luck in your studies and your pursuit of admission :)
How long will you need to study for? There is no exact magic number here. Some people can score a 328 cold in their sleep. Others will study for years and never get close to that. I guess what we need is a baseline. So go ahead and a powerprep test, or two, and see how you do. If you're already close to your goal, you might not have to work that long. If you're very far away, it might take awhile.
Luckily, quant is easier to improve than verbal, and progress tends to come quicker. This is because it's pretty linear: today, you might not know exponent laws so you get exponent questions wrong. But then tomorrow you can learn the exponent laws, and no you won't miss those questions anymore. So bit by bit you can just learn what you need to learn. The creative application of concepts, critical thinking, and logical problem solving skills are harder to learn, but it seems like you probably have pretty good intuitions in those areas.
But start with the practice tests and see what your baseline is. Stick to official stuff at first and see how it goes. If you need more you can look to outside sources.
Let us know how things go! I'm here to help.