The Tueller Drill teaches that an attacker can cover 21 feet in 1.5 seconds, which is longer than most people can effectively draw from concealment.
If an armed belligerent is within about 21 feet, you should draw and issue commands to drop the weapon. If he instead advances toward you, you should shoot to stop. Even if you shoot an attacker at 21 feet, if he's hyped up sufficiently, he can still close the distance and stab you before he drops, so once you start shooting, you should keep shooting until he drops.
The question becomes, “how does one adjust to the 21 foot, 1.5 second situation?”
Simple. Where possible, backup while drawing your weapon, thereby giving you more valuable time!
In such situations, it is incumbent on the attackee to be aware of their surroundings. In a possible other situation where it may work, turn and run until your weapon is drawn. Then defend!