Actually, the "waiting period" may be up to how your nurse aide registry operates and/or what you wish to do.
For example, in West Virginia, their NAP policy manual recommends candidates test at their earliest opportunity. Candidates have test eligibility status for 24 months unless they want to challenge; in which case they have six months.
If a candidate does not take and pass the test within their 24 month period, they have to complete training again.
However your state does it, getting it out of the way as soon as you can while the content is fresh in your memory is just good practice too.