I used Virtual ATI and I passed NCLEX. I can tell you that I was signed up for the 6 week program and I took my NCLEX before the 6 weeks was up. That being said, I devoted more than 2-3 hours a day, so sometimes I was doing a new test every day (when my instructor would send me the codes....)
I did more than just ATI for studying as well.
And fyi, my school used ATI, too, but Virtual ATI is NOT free... It's $250.