That is a moving target.
DFAS normally pays retro CRSC within a 2-6 month range after they receive the CRSC approval document. Recent retro payments have been in the lower part of that range.
Cases with a retroactive effective date for retirement approved by the Physical Disability Board of Review (PDBR) can take much longer.
Opinion: In general, I suspect the depends on the volume of cases they receive during a certain period.
Ron