The 21 foot rule (7 yards) is taught heavily by the National Rifle Association (NRA) and other firearm instructors. The premise is anyone within 21 feet can reach you quickly with a knife, club, or their bare hands. The 21 foot rule was established after several controlled drills which assumed an assailant must be within 21 feet to cause great bodily harm or death.
Ready-Aim-Fire in 2.5 Seconds
While the study showed a trained police officer could draw and fire their weapon twice before being physically attacked, the test was conducted under "ideal" conditions where actual threat of bodily harm did not exist. This makes the study less likely to work in real world scenarios, where tensions are high and adrenaline is rushing through the victim's body. Based on the results using highly trained police officers, it was determined it takes 1.27 seconds to assess the danger, un-holster a firearm, take aim, and hit center mass; the likelihood of an average citizen being able to do so efficiently and accurately is extremely thin.
Attorney's Use the 21 Foot Rule for and against the Victim
The 21 foot rule has also been grossly miss-used as a standard for a justified shooting and it's been used as a double-edged sword since its conception. If the attacker is 21 feet away but has not moved towards you, prosecutors argued your life was not in imminent danger. If the attacker moves towards you closing the gap, defense attorney's argue this implies an automatic acceptance that you can use deadly force. However, depending on witness testimony and your own account of the situation, the 21 foot rule could be your undoing if not articulated correctly. Which is why you want to exercise your Constitutional RIGHT to have an attorney present during questioning. This is your RIGHT; assert it!
The 21 Foot Rule Re-tested
When tests were performed by other reputable companies, it was determined the average time a trained police officer could pull their holstered weapon, aim, and fire two shots was between 1.7 and 2.5 seconds. The test was performed by trained police officers under laboratory conditions--without the actual fear of real imminent danger. The time it took the assailant to reach the police officer was between 1.27 and 2.5 seconds depending on the physical condition of the "attacker". The chances you can draw your weapon in the same amount of time as a calm, un-threatened, trained officer is clearly questionable.
Concealed Carry Limitations not Considered
The test does not take into account a concealed weapon carriers response--the tests were conducted using a firearm holstered externally on the officer's strong side shooting position. If you consider the time it takes to access a concealed firearm, the test fails every time. For example, if you have an inside-the-waist holster, time is required to clear your clothing, grip your firearm, and point it in the direction of the attacker. If you have an inside-the-pocket pistol, time is required to reach inside your pants, pull your weapon, and aim it in the proper direction. The 21 foot rule simply does not provide ample time to perform these additional steps as a concealed carry citizen.
Keep in mind the 21 foot rule was created for trained police officers dealing with a suspect armed with a knife. Officers are trained for these situations, law abiding citizens are not. In all cases, talk with your attorney before you are faced with the need to draw and fire your weapon to get a complete understanding and legal opinion.
There's an excellent article on Police One which delves much further into this subject.