I recommend you take a practice GMAT test, try the one from GMAC. They are computer adaptive and you can take it multiple times as a result. Skip the AWA and once you are finished go through your errors and create an error log
to see what you messed up. Look for patterns. For example, in my verbal 6 out of the 7 I got wrong were Sentence Correction. So it was clear where I was lacking.
How long it is going to take depends on how many topics you need to go over. I would say roughly 2-3 weeks of straight quant, 2-3 weeks of straight verbal, and then 2 -3 weeks of practice tests and going over your problem areas, finally a week or so of basic AWA practice.