I've been involved in many flight test programmes in the past albeit most without Boeing's resources. 9 months for a brand new type strikes me as being sporting at best. It only takes a small snag to delay things and the whole programme falls apart. A simple flight test incident, a result you don't understand, a dependency nobody spotted, all can lead to delays. I don't know how many test aircraft are involved in the programme but that itself brings problems with mod standards etc.
With the current delays, however, they may have been able to carry on with much of the essential preparation work.
They are obviously planning for success with no "fat" in the programme at all. Good luck to them - I think they'll need it.