First - I know this isn't what you want to hear - but it's extremely unlikely that you're going to get to 680 in 15 days. That's not nearly enough time to jump from mid-500s to 680. Whatever you're scoring right now is roughly what you should expect to be scoring 2 weeks from now.
Again, I know that's not what you want to hear, but it's better to hear that now so that you can make some decisions for yourself. You're most likely looking at several months of work, not several weeks.
How have you been studying so far? What resources have you been using? Read the below article; have you been studying in these ways? If not, start doing so.
http://www.manhattangmat.com/blog/index ... an-part-1/Next, I'd like to get some more detailed data on your strengths and weaknesses in order to advise you. Use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Then come back here and tell us the results of your analysis and what you think you should do based on that analysis. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
You probably have timing problems - almost everyone does. Read these two articles and start doing what they say:
http://www.manhattangmat.com/blog/index ... to-win-it/http://www.manhattangmat.com/blog/index ... nt-part-1/It typically takes people 4 to 6 weeks to fix timing problems.
Oh - and did you take those practice tests under 100% official conditions, including essay and IR, length of breaks, etc? If not, then your practice scores could be artificially inflated - yet another reason why a jump to 680 is unlikely in 15 days. :(