GMAT Study plan

OK, so, since your goal is to finally completely prepare for the GMAT and you have already prepared some, probably your best move is to study on your own at your own pace, possibly using a self-study course. After all, going to a live class that will cover things you already know or cover things in not that much depth is not going to fit your situation.

Say, for instance, that you use a self-study course, you could go through the course normally, and when the course covers anything you find you already know, you could skim the material and answer enough of the practice questions to be sure that you have that topic down. On the other hand, when the course covers things you aren't familiar with or are not already pretty confident in, you could read the material thoroughly and do all the practice questions provided.

The key thing is going to be for you to really dig in and complete the project this time. Accordingly, you'll want to master each topic before moving on to the next, review previously learned topics regularly, and in general be thorough in your preparation.

Regarding how many months you'll need, that depends on how much you have to accomplish to get to your score goal. So, perhaps, after you have gotten back into preparing for a week or two, you could take an official practice test to see where you stand.