Decision to QUIT job..GOOD or BAD

Not having a job is a not a great point on your resume. You do need to show professional growth, career advancement, and leadership skills since that's what MBA is about. Also, your letters of recommendation will be weak since the people writing them would have known you only for a few months or would have been your former bosses.

As to studying, assuming you speak English well, you should not need more than 3-4 months @ 2-3 hours a day. That's how much I have spent and was able to improve from 540 to 750. I think you sould be able to do it too as long as you do it early in the morning when your mind is still fresh and you are not distracted/multitasking.

P.S. I don't think you should be unemployed and just professionally preparing to the GMAT - most of us could get 800 that way if we were able to spend our days flipping books and having no pressures or stresses from work.

There is no reason you should work at a place you hate - you are welcome to what you love and what your calling is, but you can't just be a GMAT bum, not for US schools at least.