I used to be a Kaplan tutor and instructor. Now I occasionally tutor students individually. It’s been my experience that if your aptitude is “average+” and you want to score above a 700 you need about 30 hours of instruction and then about another 50-70 hours of studying on your own. This seems like a lot but to some of my students it has been a slam dunk way of scoring 700+ by studying close to 100+ hours.
Data from GMAT, the creators of the GMAT, bolster this positioning. 700+ scoring students study 114 hours according to GMAC themselves.
I would definitely start studying 3 months out. This means taking a class course or tutoring if you have the money. After the class is done I would recommend taking one practice CD test a week (Sunday Afternoons).
During the week, stay after work for 75 additional mins and take a verbal section or a math section on CD.
The class course should teach you how to approach each problem. The tests and CDs should teach you timing.