Hello,
Though I am not an expert, but hope attempt with my opinion can help you out.
some suggest that by attending more Mocks, is to make sure, you complete the test well within time, understand the level of difficulty of question stem, and find your flaw to improve ahead, for some MBA.com two free test taken at certain period of time is sufficient, but for others, to gauge themself time to time, more than 2 or 3 Mocks is required.
650 and 680, are one of the good score considered in GMAT, which you have achieved well within time, and surely you can improve more, with time you have till the D day.
Yes, there are some free Mocks available, based on sign-in Eg.
Manhattan -1
Priceton - 1
E GMAT Sigma - 1
Kalpan - 1
GMAT Club - 9 Verbal and 25 Quants
Vertias Free - 1
And Best of Best is MBA.com
1 and 2,
3 and 4, and
5and 6.
*Precaution - don't jump in all at once at them, go one by one with more understanding.
Plus to practice more difficult question, you may purchase additional approx 800 question directly from MBA.com, which will strengthen you and bring you more close to actual question pattern of GMAT.
For time management, there is a good article written by experts of GMAT club, see if it can help you out.
Relaxation is the only state of mantra : to focus on your objective to achieve.
Lastly don't lose your consistent with looking at current graph improvement, practice on fundamental more and remaining with time left on problems, because problems can vary, not the fundamentals
Wishing you all the best for your future
Regards
Posted from my mobile device