Once the FAA has given it's sign off then Boeing will act aggressively to get the fix installed on the 50 787s that currently in customer hands. I would expect that there will be multiple Boeing teams at different locations working concurrently to do the retrofit which, as mentioned earlier, will take 4 to 5 days per airplane.
Much has been made that the root cause of the battery incidents has not been discovered and that Boeing and the FAA are rushing this. However, there is precedent for this type of actions since the evidence pointing to the root cause was probably destroyed. When TWA Flight 800 exploded of Long Island's South Shore there were many theories that abounded but the NTSB pinned the blame on fuel vapors in the center wing fuel tank that were ignited by a spark. There was no direct evidence of that but the result of their findings was that the FAA required some sort of fuel inerting systems to be retrofitted and that new aircraft designs have a an inerting system designed into the center wing fuel tank. All this on an aircraft accident that did not have a root cause. Here Boeing is taking action that would address any potential failures in the battery and severely mitigate the risk f a fire from ever happening by siphoning off any oxygen surrounding the battery.
Thus with its reputation out on the line, Boeing Commercial Airplane Head, Ray Conner and Chief Engineer, Mike Sinnett explained, in detail, Boeing's proposed solution and why they have confidence in it. The briefing in Tokyo was very technical in nature and very through. Here Boeing revealed that they expect that they can be done testing and start implementation of the fix in weeks and not in months. They qualified those remarks by stating that the FAA has the final say in lifting the airworthiness directive that lead to the grounding. One bit of news that is interesting is that Boeing will not lose the 180 ETOPS certification once the FAA has signed off on the fix. They still intend to achieve ETOPS 330 for the 787 though it is uncertain if the FAA will add any more tests for that certification because of the battery issues.
Boeing is rumored to already be producing parts for the retrofit on the 50 aircraft that are in customer hands as well as the 25 siting at Charleston and Everett as well as the aircraft that are in various stages of production. If Boeing can meet the schedule it has laid out to the FAA and get the testing done in weeks, I would expect the FA to take it's time in analyzing the data and perhaps ordering further tests. The 50 787s may not resume passenger flights for another 4 to 6 weeks (that includes the retrofit time). Deliveries probably will resume around the end of May but it ll depends on the FAA and how stringent they plan on being.