It took me a little over 3 months to study for the GMAT and I got a 700+. My recommendation to anyone who's just starting is to give yourself at least 3 months, purchase all 7 of the Manhattan guides + 3 official guides and just start hammering away. Also, those who have already studied for the LSAT (and I know I wasn't the only one), the CR and RC will be a breeze. But for those who haven't, I wouldn't go so far as to *practice* on LSAT CR or RC, as those are far more difficult than the GMAT. Instead, it would be good to pick up a PowerScore CR
LSAT guide just to familiarize yourself with the different question types for CR questions. From there, just hammer away on GMAT CR questions and over time you should be fine. As for RC, I really don't have much to say there -- it's similar to any SAT, ACT, etc... exam, so try to brush up on the main point, author's opinion, etc and you should be able to move along.
Personally, the hardest part for me were quants. I really just spent a ton of time familiarizing myself w/ the different question types and memorizing the few formulas that you need to get by. The Manhattan CPU *scores* were fairly accurate, but one caveat is that their quant sections was insanely difficult. After the first 2 or 3 exams, I gave myself more time. If you're just starting, one thing that may help is to increase your time by 15 minutes on the first 3 CPU exams, then only by 10 minutes on the next 3, then only by 5 minutes on the last 1-2, then go real-time on the GMAT provided exams (which are VERY accurate). Some people will argue with me that this won't reflect an "accurate" score, but the quants section on the Manhattan CPU exams aren't even close to the accurate. Furthermore, by adjusting time to reflect the increase difficulty, it provides you an opportunity to work through problems (instead of guessing). That way, while taking the exam you'll be learning the fundamentals (although it will never reach that level of difficult on the real thing).
That said, good luck!