Shooting instinctive still requires a solid anchor, that is a requirement for any style of shooting. The time it takes to release the arrow is dependent on the time it takes the shooter to focus and get everything together. The shot can be and will be more fluid through practice.
Like anything learned, practice is taken one step at a time. There are times when my shot appears to be released before I reach anchor or just as I reach anchor and there are times when I hold a full two to three seconds after hitting anchor.
When you see a person release before they hit anchor, they are snap shooting, which is not instinctive shooting. It is a breakdown in the shooting form, which leads to target panic, and in most cases is due to shooting a bow too darn heavy in the first place.
While there are a large number of people who perfer other shooting methods, I find the instinctive method a very good one for me. I think your question is a very good one, because many don't understand what it is.
Form, form, form, is the key. Don't shoot too heavy, find a good anchor, it is your best friend, and take your time on the shot. Regardless of the style you shoot, I agree with Byron Ferguson, you should be able to hold your bow at full draw for ten seconds and still get off an accurate shot.
Regarding your longbow, proper preparation is important. You have selected your stand site, you should trim it correctly. When I place a stand or blind, I take the bow I expect to shoot with me into the stand to make sure nothing gets in the way. Learning to shoot your bow in different positions will also help.
Remember shooting a bow instinctive has nothing to do with how fast you shoot. The advantage in my mind of shooting instinctive is the fact that I don't have to worry about distance, or some sighting system, it allows for me to totally concentrate on the spot I want to hit alone, and to totally enjoy the flight of the arrow. That flight is why I got into shooting a bow in the first place....