I know lots of people that have took the NAPLEX without studying much at all.
With that being said, it's really going to depend on the quality of your education and how much you've retained... as well as how well you do on standardized tests. Some people just don't perform well on these kind of exams because of the pressure or the format or whatever.
As far as the 30 days goes, I don't think that's too late to start studying, you just may have to be a little more intense than if you had started earlier.