That's a personal thing that is slightly different for each individual. If you are curious enough, get Byron Furguson's "Become the Arrow". It's a good read. I use his method mostly. But for longer shoots, I use a gap method.

In short, I concentrate on the target, while drawing, aiming, and following through. I keep the arrow in my periferral vision (but never looking directly at the arrow)directly under the pupil of my right eye.