I would imagine they would need a 6 month duration to get exec approval, followed by 6 months of testing to roll it back :p
I would imagine the opposite - that long before the rollout started, the rollback mechanism was tested. Checking this part of the process will have been part of the acceptance testing.
In the very earliest stages of the rollout, while being watched carefully, rollback would have been an integral part of the daily plans. People responsible for an overnight upgrade will have needed to decide, there and then, whether to rollback, while the execs would only find out what happened after breakfast. I have the "been there, done that" T-shirts for such events - from one memorable occasion of taking out a chunk of SE England before rolling back (embarrassing), to another of having to report back to the execs at breakfast time why I needed to stop a widespread software release from happening (scary). In the last one, I had about 3 hours to call on other technical staff to check that I wasn't mistaken, and that we really were about to release something that had gone FUBAR.
As trust is gained through the small, initial rollouts that run just fine, the plans for widespread rollout will tend to accelerate the rollout at the expense of room for rollback - as it had already proven unnecessary.
The problem, in this case, is that the rollout became so widespread before the ill effects were noticed; rollback then becomes non-trivial. By this point, it does need exec approval because there is likely to be a significant cost to choosing to do anything - stopping, continuing or going backwards.
The first question the high-up exec will be asking: What is the scale of the problem? That "left hand - right hand" problem Kitz mentioned suggests that the answer to this question was not initially known.
After that, the question is whether to fold, stick or twist. In the absence of understanding the scale, a "stick" seems like the easiest first choice.
I know people like to characterise organisations like BT as full of incompetence as well as bureaucracy, but it really doesn't work like that.