I think if you attempt to maintain intense focus for 18 holes, it would become diluted over time and also very fatiguing. I agree the answer is preshot routine. In my case, I have a roughly 10 foot circle that when I step inside of, I am beginning the execution of the shot.

The simple act of removing the club from the bag is my signal to renew my focus. There is an old saying in archery that "the shot begins when you remove the arrow from the quiver". As I wait to enter the 10 foot circle I normally visualize the shot and might take a practice swing it it does not disturb someone else. Once I step across the 10 foot line the shot begins. Outwardly this would appear to take little time. As Billyg mentioned, 30 seconds or so is enough.
Cheers,
Bob