HI Amaral_rui,
A December Test Date might give you enough time to hit a score goal of 700+, but you haven't provided much information about your situation, so it's tough to assess your chances.
1) Do you have any prior experience with the GMAT (have you studied? Have you taken the GMAT before?)
2) Do you have an Official Test Date scheduled yet (and if so, then what is it?)?
3) When are you planning to apply to Business School?
4) Have you taken any FULL-LENGTH CATs yet? If so, then how did you score (including the Quant and Verbal Scaled Scores)?
5) How many hours do you think that you can study during a typical week?
GMAT assassins aren't born, they're made,
Rich