I work twelves also, spend half hour with the family then study for an hour before bed, for about 4-6 hours on days off and the longest I have studied is two weeks, the shortest was six days.
I think everything has to be taken into consideration so the answer is different for everyone.
A LPN with one year of experience may have to study longer than someone that has been doing it for several years.
A LPN that has only worked in one place/specialty for their career may have to study longer than someone that has moved around to different jobs/areas of specialty.
An EMT may have to study harder than a LPN because they don't learn the nursing process so there is more for them to wrap their brain around.
Some people, no matter how smart they are, are TERRIBLE test takers so they study longer.
I guess what I am trying to say is don't base your study time on what others have done. It's such an individual thing that if you study for two weeks and arent' confident you may feel you are doing something wrong, but you aren't.
When you get started take Part one of the practice exam, study your weak areas, take part two of the practice exam, when you feel confident sit for the actual exam. The good thing is you came to the right place, we are all here to help!