This method is the one I use as well. We all hope for a relaxed, unsuspecting animal. In most cases, I'll wait as long as practical, taking into consideation these factors;

1) Any sound my clothes may make during the draw. Especially in cold weather.
2) The bucks approach to my shooting lane and his general body language.
3) Terrian issues that predict how much time I have to effect the shot.
4) Shot angles that could change if given too much time after the draw.
5) Any natural cover that will conceal the bucks eyes as I draw my bow.
6) How high I was positioned, if I was tree stand hunting.

If the animal is hyper, I'll try to give him time to relax. Otherwise, I'll let him walk if the circumstances spell an unethical shot at a spooked deer. I have harvested a few bucks on the edge of knowing something was up... but these situations have a case by case set of rules to draw and shoot by. Each one can differ greatly, which makes it super hard to give a definitive answer to your question. I would error on the side of not sending an arrow if I was the least bit sure of making a humane harvest.

That reminds me... I shot a nice 8 pointer that I drew on when he was almost at ground zero... he was preoccupied with another buck who showed up at the same time. He missed me drawing back and looked up at the release, millaseconds from the arrow zipping through both lungs. Recovery was 35 yards. He was relaxed, coming in to see the smaller buck or I would never have attempted that shot. I believe he would have busted me otherwise. If he's REALLY on edge & acting skittish, simply don't shoot.