An average human can cover 10 yards in a second. I try to teach a minimum standard for drawing from concealment to first shot of 1.5 seconds, with a goal of 1 second flat (and faster with practice). With that said then, anyone with a knife withing a 10+ yard radius of you is an immediate threat (meaning you will barely be able to draw fast enough to defend yourself). I'd say a person 20 yards away or more might even pose a threat. What this boils down to is that anyone wielding a knife as a weapon anywhere in your vicinity is too close. Obviously you need to evaluate the duty/ability to retreat before engaging such a threat, but the reality is that a knife wielder is dangerous from much farther away than people realize.