Given we are in the 21st century, is it not to be considered unacceptable, that the authorities, given to them power to investigate, cannot with all the evidence laid in front of them come up with an answer as to the cause?
I would suggest 7 days, 10 at the most is adequate. If the AIB need 12 months plus to resolve the reasons for any accident/incident I would suggest they are sadly very lacking, and not fit for service.
Possibly they are financially capped, short of personnel, these issues must be resolved.
It is not acceptable in this day and age that an incident is not resolved within a very short space of time.