There is no "eligibility" for a MS. You can get one right out of your bachelors or wait 40 years.
Your prep time depends entirely on how much you're willing to allot for preparation. Some people do 6 months, some 4, some are still in school and do very little. Personally I did 2 months and thought it was fine. Could I have used another month? of course. Anything less than 2 months and I probably wouldn't have felt as confident during the exam.
There are several prep guides that some people like more than others, Magoosh, Manhattan, Kaplan, etc. Read reviews in the forum.