Hi backthatassetup,
Studying 12 hours a day for ANY extended period of time is a BAD idea. There's a threshold at which you simply won't be doing anything beneficial - your work will get sloppy, you won't remember anything, you won't be able to stay focused, etc. For most Test Takers, that happens after 2 or 3 hours of straight studying. As such, if you DO want to study for a longer period of time on any given day, it's a good idea to "break up" your studies into "chunks." For example, you might do 2 hours and then take an hour off....then come back and do another 2 hours and then take an hour off.
Since you're asking about this, I have to assume that you're seriously considering this approach.
1) When are you planning to take your GMAT?
2) What is your score goal?
3) Why would you try to do so much in such a short period of time?
GMAT assassins aren't born, they're made,
Rich