When a bank merger takes place how long does a customer have to cash in a cd without penalty?

What happens when banks merge?
If an account owner has deposits in Bank A and Bank B and Bank A merges into Bank B, deposits of Bank A continue to be insured separately from the deposits of Bank B for at least six months after the date of the merger. CD's from Bank A, the assumed bank, are separately insured until the earliest maturity date after the end of the six month grace period.

This is from the FDIC's website

And, from Regulation D, a waiver of the early withdrawal penalty is permitted --
So, if the six-month grace period allowed by FDIC rules has expired, but the CD is within 6 days of an earlier deposit or withdrawal or renewal, the penalty can be waived if the CD puts the depositor over the limit for FDIC coverage.

Pretty slim chance, I'd say, given the short mandatory penalty window (six days) and the advance planning time afforded by the FDIC.