My experience was not typical. I would have waited if I had it to do over, but.. (I did just recert by exam last fall - I was much more comfortable with the questions!)
I did the CEN after 4 months of full time ED practice. I was the first in the department and several others have followed since.
But, I had been a medic for 9 years at the time and had taken TNCC and ENPC prior to the exam (just after I was licensed). I also had been teaching in a paramedic program along with ACLS and PALS for a while.
RIGHT NOW you should focus on your clinical practice. Get involved with your local ENA chapter (networking, educational options) and then look and prep in a year or two.
Good Luck!