I lodged my application to QNC december last year. I received my first assessment with my documents from them last week of February. I had only one document lacking which I sent on the same week, but received by them on the first week of March. Then a week after, I had the approval to take the CAS. The definite reason why most applications take long to be approved is that the documents are not submitted completely during the initial application--like IELTS, verification of initial registration, NBI, evidence of recency, competence and fitness to practice, etc. Mine just actually took 2 months.
Anyway, right now, I'm just awaiting for their reply regarding my intentions to proceed for the last written exam this April, which is already next month (Oh oh!). They already had my receipt, but, in their letter, they reinforced about the current status of CAS in relation to the national registration of Nursing in Australia from 1st of July. The letter served as a basis for applicants--those who were approved and can still sit for the last written exam, like me--to choose whether to proceed with CAS or wait until the imposition of new requirements for national registration.
Don't worry, just wait patiently for your assessment. I believe QNC and CAS have been working out for the extension of registration time of the existing applicants. :)