4-6 hours a day for 2 months should be plenty, but it might be not enough or it might be too much, depending on your individual circumstances. I'd suggest starting with a diagnostic test, to figure out where you stand. The Official Guide questions will suffice, if you want to save a little money. There are several basic types of questions in the GMAT, and the OG can help you figure out which of these questions you are able to handle, and which you need help with. Once you know which types you need to work on most, focus on that specific area, and then move on to the next weakest, and so on. Keep taking practice tests every week or so, to allow you to keep track of your progress.
Additional material can be helpful, as can online courses or live classes; again, however, this would depend on your individual circumstances. Some people respond well to certain types of study aids, while others do best without any additional kerfluffle. You know best what works for you. What is certain, however, is that there are plenty of people here who are willing to help you - feel free to ask about anything and everything you might be having trouble with!
Hi, I'm DonQuixote, a former GMAT Pill student who scored 780. I'm very grateful for this score and have now joined their staff My account of my GMAT experience can be found here.