Here is the Chinese copy of the CAAC/EASA certification agreement if anyone is interested in knowing how it works.
In case of C919, 审定当局 (certification authority) is CAAC, 认可当局 (acknoledge/acceptance authority) is EASA.
Chapter 4.4.4.2
Acceptance authority shall issue certification when all of the following conditions are fullfilled
- Certification authority has issued its own certification.
- Certification autority has provied evidences that the product fulfills chapter 4.4.3
- All extra issues that Acceptance authority raised during the acceptance phase have been addressed.
- Applicant (COMAC) has fulfilled other requirements.
Item 3 is EASA's work right now. Assuming CAAC and COMAC work proactively, they would have provided their documents to EASA during CAAC's certification process, EASA would be able to conduct its acceptance procedure almost simutanously with CAAC's certification prcedure. I don't know how long (extra) time EASA needs to finish its part, but can't be years.
Also from the agreement, CAAC would be conducting acceptance procedure for any future Airbus types. So I think it is to the best interests of both sides to keep it to professionalism.