550 to 700 in 75 days?

150 points is a big jump, but I've read stories on this board about even bigger jumps. So it is possible, and if you want to achieve it in just over two months, you need a solid study plan. Why do you have to take the test in 75 days? If this is flexible, you have a better chance of achieving the score jump you want. Do you have a solid study plan yet? This is what we recommend, time-wise:

In terms of how many hours per day to study, you should look at how much time you have available. We suggest that you study for at least 1.5 hours per weekday and 4+ hours on weekend days (assuming you have a traditional work-week schedule). You should also pay attention to your personal needs. For example, if you find that 4 hours of study is too much and you are not absorbing information, then reduce the number of hours you study at one time. It would be counterproductive to continue studying if you’re too exhausted.

You should also take short breaks after each hour of studying to clear your head and renew your energy. Ideally 10-17 study hours per week is good goal, but this can easily go up or down based on your availability and how soon your test is coming. Part of the your study time should also involve reviewing summaries and notes you take along the way. In total, it’s ideal to allow 8 to 12 weeks to study properly for the GMAT.