I can't agree more with what was said above. My first diagnostic I scored low 500s. After about 80-100 hours of studying, I consistently scored into the low 600s. It took me maybe another month and 100+ hours to score into the high 600s, trying to break 700. My goal is to score as high as possible on the real exam 5 weeks from now.
In terms of my daily study habits, here it is. I hope that it offers some direction:
A little about my daily life... I work full-time about 45-50 hours a week from Sunday through Thursday. I am President of an organization which takes about 5 hours a week of my time, depending on which events are coming up. Finally, I also spend some time in the gym to "get away." Otherwise, I'm always studying.
On work days (Sunday-Thursday), I am studying from 2-5pm. I am going over OG questions, other problem sets, reviewing strategy guides if I feel that I need to. On Thursday, I prepare to take a 3 hour CAT exam.
On weekends (Friday-Saturday), I am spending roughly 8-10 hours on reviewing my CAT exams thoroughly and to set up my schedule for the following week.
I hope this helps a little bit. If you want to speak more about which schedule would be best for you, let me know. We can chat online or something.
Don't let a low GPA destroy your dreams of a business education.