This is directly from “the horse’s mouth” an article by Dennis Tueller from the March 1983 issue of SWAT magazine.
The article addressed Tueller’s own experimentation, which determined that “the average healthy adult male can cover a distance of seven yards (21 feet) in about 1.5 seconds.”
And this from The Tueller Drill Myth that also explains it is just an average, as @Kevin29 also pointed out.
The significance of the time factor is based on the reasonable standard that a person who’s trained in proper pistolcraft should be able to draw a handgun and place two centered hits on a life-size silhouette at seven yards in about 1.5 seconds. Before I go any further, I want to point out that both the distance of 21 feet and the time factor as addressed in Tueller’s original article, were both approximations based on training experience; nothing more.
The other point to understand is that this is the average distance/time in which it is a tie, wherein the attacker is upon you and you are able to put a shot on target. Not a situation anyone would want to be in. Both articles I cited also mention what we always address - awareness and avoidance, do not wait to be assaulted to move. As Kevin Michalowski says, “get off the X”.