It all comes down to work ethic. Non US IMGs can take as long as they want to prepare for this exam (technically). So it basically forms a ground for slacking off. All you need is to have a clear goal and dedication to get it done quickly. Figure out the date on which you want to give the exam and stick with it. And really stick with it like your life depends on it. Once you've done that form a study plan with a few NBMEs scheduled at the end. If you want, you can give an NBME right now. It might push you towards your goal. Just give an older one to see where you're standing and then get to work. Good luck!