Hi jerkatissm,
To start, a 680 is a fantastic initial CAT Score (the average Score on the Official GMAT hovers around 550 most years), so assuming that you took this CAT in a realistic fashion, this result shows that you're a naturally strong critical thinker - which is good. As such, you might not need a long study timeframe to hit your Score Goal - but you will need to be consistent with your studies. Many GMATers spend 10-15 hours (or more) each week on their studies. While you might need more or less time than that, if you're really interested in a 750+, then you have to respect that approximately 98% of Test Takers never score that high on the Official GMAT (regardless of how long they study or the number of times that they take the Exam). By extension, you might need to invest more time and energy that you initially think that you need to.
1) What were your Quant and Verbal Scaled Scores for this 680?
2) When are you thinking about formally beginning your GMAT studies?
GMAT assassins aren't born, they're made,
Rich