Google Dennis Tueller. He was a police trainer that was asked at what distance was a knife wielder a threat to an officer. After some testing, they determined that an officer with a gun in a duty holster averaged a 1.5 second draw time. An average adult male can cover 21' in 1.5 seconds.
Having played with this at the sheriffs dept and at my dojo, 21' is about right for the guys with open carry holsters with minimal retention who train a LOT. People with higher retention holsters and/or concealed carry increase their threat distance a LOT.
Easy test. Get a friend with a pointed stick or a lawn dart or anything they can jab into the ground. You stand with your gun holstered in your normal carry mode and clothing, facing a target with a good backstop behind it. Your partner stands on your non gun side facing away from you, holding the stick. A third person calls out "GO" (or use a shot timer), at the command, your partner with the stick runs as fast as he can away from you, while you draw and put a round in the target. When your partner hears the shot, he spikes the stick into the ground. Check the distance. If you assume that you have a magical death ray gun that stops them instantly upon firing, then that's the minimum distance a knife wielder can be away before they reach you to shoot. To make it a bit more realistic, have your stick wielding partner decide when to start by having him simply start running when he's ready and you draw when you see him move. Most people get a truly eye opening experience out of it. If you try out outdraw a knife, you get killed.
I've done some knife work in my martial arts training. Padded up and sparred, used blunted training drones in our street sparring modes (padding still worn, but in regular clothes and shoes, with hidden weapons accessible). Dealing with a knife threat sucks, plain and simple.
The idea of fighting with a knife against another knife wielder is fantasy in this day and age. Knives are used when the other person doesn't know the have it, either while they're threatening with a gun, or trying to bash their face in.
I recommend getting the Dog Brothers/Gabe Suarez DVD series "Die Less Often, interface of gun, knife and empty hands" lots of good drills and defenses that actually work, and let you get your gun out.
