I base the time of certification on my own experience, I have completed 8 certification so far. I am waiting for CSOP to start another certification in a few weeks.
For a domestic carrier you can easily expect 2 yrs. Eastern are using brand new airplanes, that makes it much easier since FAA does not have to go back years to review maintenance documentation.
For older airplanes all maintenance records, must be reviewed, older airplane longer it takes. for larger airplanes this would require more support from FAA for all checks, ie evacuation demonstrations.
For international carriers, this makes the situation even more cumbersome. All government employees must have state department approval, this is a so called blue passport and is not issued to all FAA inspectors. To conduct proving runs over water requires Navigational specialist to be on board, including Maintenance and Operations Inspectors.
In addition you are looking at close to 15,000 pages of documentation that needs to be produced, review and corrected prior FAA approval.
It is not easy.
