Generally speaking, no, people should not quit their jobs in order to focus on GMAT prep. While GMAT scores are important for b-school admissions, job experience and recommendations are MUCH more important.
How have you been studying so far? Are you taking a class, working with friends, studying on your own? What materials are you using? If you don't feel like you're making much progress, you may need extra help. Talk to your teacher, if you have one; if you don't, you may want to think about taking a class.
While the ideal is to sit down for 60 to 120 minutes at a stretch when you're well-rested and focused, that can't always happen. If you need to break things down into smaller stretches during the work week, that's fine.
Are you a morning person or a night person? If you're a morning person, can you study in the morning before work? Maybe half an hour in the morning, half an hour at lunch, half an hour in the evening? How do you commute to work - could you study while commuting?
Does your boss know that you're studying for the GMAT? Is it a positive or negative thing in your company for someone to get into a great b-school and leave the company? Some companies really want this to happen and may allow you to take a little time each day or a few times a week to study during the day. (Some companies even pay for a GMAT course and will arrange to hold the course at your office! We have lots of corporate clients.)
So, your overall score has mostly been staying the same. Have your subscores also mostly stayed the same? Or are they moving up and down? When did you take each practice test and what were your exact scores? Do you know what your strengths and weaknesses are - very precisely? Here's an article that can help you to figure that out:
http://www.beatthegmat.com/a/2009/09/23/evaluating-your-practice-testsAre you studying in the right way? Here are the kinds of things you should be doing when reviewing a practice problem:
http://www.beatthegmat.com/a/2009/10/09/how-to-analyze-a-practice-problemDo you have a study plan? Here's an article about how to develop one:
http://www.beatthegmat.com/mba/2010/02/09/developing-a-study-planIf you don't really feel like you have a plan, start with the last article first. Then move to the "Evaluating your tests" article.
If you want to discuss anything related to the above, come back here to update us on your situation and ask your questions!