Only you will know when you are ready.
I graduated in December '10, took about a month off, started an NCLEX review course which ended up being almost a month long. We had a lot of practice questions throughout the course as "homework" and I did additional practice questions on my own time. I took the NCLEX in March. So, I waited three months.
The key is doing practice questions!!! Get a good NCLEX book. Saunders is great and so is Hogan's NCLEX-RN Reviews & Rationales (Pearson Prentice Hall). For alternative-format questions (practice with SATA questions, etc); check out THIS
Honestly, you will never feel 100% ready to take the NCLEX, but once you start getting a majority of your practice questions correct, that's when you should take it! Good luck!