Hi VIGupta,
Unfortunately there is no hard and fast answer to your question. A lot of it depends on your starting point.
Please refer to
Step 2: Set your Target - What GMAT score do you actually need? in
this GMAT Study Plan post. It will give you a quick-and-dirty estimate, based on experience, about efforts required and likelihood of achieving certain score improvement.
Good luck with your prep.
VIGupta
What is the ideal preparation time period vs actual score expected ratio seen among GMAT test takers? I have booked my test in end of September, will I have enough time (with target score as 675+) to prepare considering 2 hr study time each day post work timing.