I'd suggest not to go by the 'amount' of time you'd dedicate for studying and instead focus on the quality of time you'd spend for studying. Since you are already at a 680 level - you'll need to put in a decent effort to head up to the low 700s (if that's your target). Have you mapped out your weak areas ? Have you thought about improving on those 'silly mistakes' ? Just do a complete analysis of where you are going wrong. This will be very helpful in isolating those core weak areas that you need to improve upon. The key is to understand how to break open these weak areas and learn ways of going around the traditional ways of problem solving. I'm quite certain you have used all the best possible materials. So don't get 'material crazy' and hunt for new materials, rather try to use the old materials with a newer understanding of the GMAT basics.
In the land of the night, the chariot of the sun is drawn by the grateful dead