I found this Q&A from NASBA’s website that should be helpful.
Question: Exam credit is good for a “rolling” 18 months. How does that work?
Answer: You may take sections individually and in any order, but are required to pass all four sections of the CPA Exam within a rolling 18-month period. Credit for any section passed shall be valid for 18 months from the actual date the examination sections were taken. Examination credit expires by section. You will lose credit for each section passed outside the 18-month period, and will have to retake those sections. Your score notice will contain any relevant expiration dates.
For example: Let’s say you passed AUD on your first try. After a couple months, you successfully complete BEC, and then pass FAR. If you were to sit for the REG section on the last day before your credit for AUD expires, and you fail REG, you now have to sit for and pass AUD and REG by the time your credit for BEC expires.
"It takes as much energy to wish as it does to plan." -Eleanor Roosevelt
Licensed CPA in IL
FAR-72, 55, 82 (Roger + Wiley TB)
AUD-66, 92 (Roger)
REG-71, 81 (Roger + Wiley TB)
BEC-77 (Roger)
Ethics- 93