^^
My post answering the question has disappeared.
4 minutes. (It's been hinted that it was a direct reference to the UKs 4-minute nuclear warning).
The four-minute warning was a public alert system conceived by the UK during the Cold War and operated between 1953 and 1992. The name derived from the approximate length of time from the point at which a Soviet nuclear attack against the United Kingdom could be confirmed & the impact of those missiles on their targets.
I knew the answer, as the sunken Vulcan bomber used in the opening scenes were the same ones my Dad Crew Chief'd in the 60s & 70s.