The most important time to focus is at the instant you release the arrow, closely followed by the instant before and the instant after you release the arrow. I think we all intend to do this, but we don't always actually do it.
I don't know long I can hold an intense focus, but not for very long, a couple of seconds at best. More than that and my mind starts to drift, whether I like it or not. I know there have been times when I have done everything right, and still miss by more than I should. I assume when that happens, I have simply lost my focus at the moment of release, and the arrow begins to drift off the target. Of course, it could also be some form error I was not aware of, but by now I’ve made most of them enough times to know when I’ve done it.
What I’ve been doing lately to keep my focus intact is to limit my intense focus to the time it takes me to draw the arrow from 1/4” away from my nose to where the feather touches my nose, which is less than a second, plus another less than a second of followthrough until the arrow hits the target. I can stay in focus for the less than 2 seconds it takes to do that. This is something I should have started doing years ago when I first took Joel Turner's course, but resisted doing for some reason which now escapes me.