I am in California, and I don’t think there is a time restriction as long as you meet the criteria. However, I won’t wait too long post graduation due to “leaking knowledge”. In addition, health science is changing all the time, the longer you wait, could hurt you in the future.
I think you should focus your effort in "knowledge" versus "clinical" refresher.
NCLEX is mostly a knowledge based examination, not a clinical examination.
As a previous LVN who went through a RN program, I felt some of my "working" knowledge actually hurt me.
The board is very black and white, while in real life, we thinks in shades of grays; and everything, and anything is possible.
For example, if a question that asks you which one of the following is the BEST/1st thing to do/priority....
and often, at least 2 of the answers are correct. Then, it is up to you to figure whicn of the answer is MORE correct.