I know this is an old thread, but it touches upon the question that all NCLEX test-takers really want to know: when am I ready?
I answered probably a couple of thousand practice questions split between the leading NCLEX study guides Saunders, Hogan (The Red Book), and Mosby. I was frustrated that I never seemed to consistently get above 60-70% in either the book or associated CD questions. I kept pushing back my NCLEX date, because I thought I should be doing better on the practice questions.
Still, NCLEX shut down at 75 questions with a "pass." I wasn't comfortable with the answers I left on those 75 questions, but apparently I answered enough difficult questions correctly.
At least in my experience, if you can average 70% or so on the big-name NCLEX practice books questions, you'll probably pass easily.