I would also try to choose a length of time to study in which you don't think you will burn out. its great if you study for 3 months (or plan to) but if you burn out in 2 it won't do you much good. and it also depends on how much you want to study each day. you might not need as long if you are pulling 12 hour days, but if you only want to do 5/6 hours it might take you more time.
that being said, I agree with the others. take it when you are ready. but if you do pick a test day (and set a study schedule) try not to push it back unless absolutely necessary, otherwise it could become a cycle of never feeling ready and continuously pushing it back. I think that is especially pertinent for you since you are taking a year off and don't have the pressure of third year to force you to take the exam by a certain date.