S41,
The simple (but probably not reassuring) answer is not the actual amount of 'assurance', but the development of a proven and auditable safety case that demonstrates that operation of the aircraft is sufficiently safe.
By 'sufficiently', I mean that the safety case shows that the aircraft meets the target laid down in MAA RA1230, which says:
The cumulative probability of the loss of an aircraft due to a technical fault and the cumulative probability of a technical failure of the aircraft (inclusive of its systems, structure and stores) leading to the death of any aircrew or passengers, should both be assessed to be of the order of one in a million per flying hour (probability of occurrence 1x10-6 per flying hour) when operated within the conditions used for the airworthiness demonstration.
How long does that take? As long as it takes to assemble all the evidence and analysis to underpin the claims made in the safety case. (I've probably got some of the detailed terminology wrong here, but I think I'm fairly close).
If the original Boeing and USAF documentation isn't available (and let's be clear, there's a decent possibility that it never existed in the first place) then the PT is having to develop new evidence based on analysis and/or read across from other airframes. As Tuc so often (and rightly) reminds us, doing that after the aircraft has been built and flown for many years is not an easy task.
Hope this helps
Best Regards
Engines