Unfortunately, if you're targeting Northwestern your GMAT would put you in the bottom 20% (class of 2016). That does mean that 20% of the people they enrolled fell into that category, but if there isnt anything else outstanding about your application that's a definite weak point and one that you have the power to change.
My work week is very demanding and when it came to studying I took this approach.
Saturday/Sunday - 3 to 4 hours per day
Summarize after the weekend what I struggled with most from those areas.
Monday - Friday try to study approx. 1 hour per day as a review of what I studied over the weekend (and previous weeks) and master what I had difficulty with. I rarely introduced new materials during the week as my brain was pretty fried.
In total I think I studied between 100 - 120 hours. It made a huge difference.