I am an MBA, and have over four and a half years of experience. I am planning to do CMGA. I would like to know how many hours of preparation time, on average, are required for CMGA.
Well, that's a very good question. The answer has to be, as you probably guessed, "it depends". Let me explain...
CMGA, while it's only one exam, covers three papers from the syllabus - E2, F2 and P2. This means that, assuming you have not covered any of the material from these three before, CMGA requires three times as much study as any 'normal' CIMA paper. This is, of course, a worst-case scenario.
As an MBA, you are likely to be familiar with some or more of the topics. The more you've done before, the less work you will have to do. As I've mentioned before, CMGA is only the best option if your MBA had a strong finance content. If not, you may well be better off taking the conventional entry route. The more finance content you studied in your MBA, the easier you should find CMGA. Also, it depends on how good a student you were when you studied your MBA, and how long ago it was. You also have to consider how relevant (to the three papers) your work experience has been. Oh, and it also depends whether you are a native English speaker, and whether your MBA was taught and examined in English.
CMGA, while it is a 'fast track' for entry into the CIMA qualification, is definitely not an easy option.
Any one of the normal CIMA papers has a rough guideline of 200 hours study time. If you are lucky enough to attend lectures, each hour in the classroom (because of the 'added value') is probably worth three hours of self-study. There aren't many courses available for CMGA, however, as the number of students attempting the exam seldom makes it cost-effective for colleges to run them.
I would guess, therefore, that you are likely to need somewhere between 200 hours (the guideline for one normal paper) and 600 hours (if you covered very little of the P2/E2/F2 syllabus in your MBA). Remember - CMGA is not 'easy', and the more time you can spend on it, the better prepared you will be for the CMGA exam, and the exams that follow it (Strategic Level and T4). You shouldn't be aiming to 'scrape through' CMGA - that would just leave you ill-prepared for what follows.
I'm sorry I can't be of more help. I would need to know far more about your education, your MBA, and what work you've done.