20 minutes according to a news report I read when the other day. Shooting was about 10 minutes into the call the article said.
But of course those of us with two braincells to rub together know that 911 is a communications system of variable efficacy, NOT a matter transporter or a cloning machine.
Assuming fanatical zealousness on the part of the police where you live:
1. Your assailant has to ALLOW you to get to a phone, ALLOW you to dial 911, and ALLOW you to communicate coherently and effectively with the 911 operator(s) (more on THAT later). I wonder how many people (nevermind elderly women) can calmly converse on the telephone, providing relevant information, while somebody is trying to break in... or chasing them around the house with a butcher knife?
2. 911 has to be WORKING. Sometimes it ISN'T.
3. There has to be someone on the other end available to take your call. Sometimes there isn't. Ready to outrun that knife wielding maniac a little longer while you're on hold?
4. You may have to talk to MULTIPLE 911 operators. A few years ago, I called 911 to report a man passed out in the gutter in front of a bar in Lakewood, Ohio. After calmly and accurately providing the relevant information to the 911 operator, I was transferred to ANOTHER 911 operator to whom I had to provide the SAME information all over again. Again, not a big deal while you outrun that knife wielding maniac, right?
5. You have to get an intelligent human being on the line at 911. The SECOND 911 operator with whom I talked during the incident with the unconscious man interpreted my clear description of him being "unconscious and unresponsive in the gutter in front of X bar", as him being "on the roof". After wasting a good deal of time explaining that the victim was neither on the roof nor had he been dragged into the storm drains by giant radioactive ants, Kelly Bundy at 911 was apparently FAR more interested in my personal contact information than in the condition of the subject.
6. 911 has to relay your call to the police, and do so ACCURATELY. If they give those fanatically zealous police the wrong address, you'll be running from "Michael Meyers" even longer. If they don't notify the police AT ALL, perhaps because they want you to put the wouldbe murderer on the line (which actually happened in Detroit) first, you've got a LOT of running to do. Getting tired yet?
7. There have to BE police available to answer the call. If there are only enough cops available to take care of your "Michael Meyers" problem, somebody else's "Jason Voorhees" problem, or a third person's "Leatherface" problem, but NOT all three, whom should they pick, and what recourse do you have if you lose the coin toss? Here's a hint: NONE.
8. The police have to be not just zealous, but competent. They have to be able to follow instructions and read addresses.
9. ALL of that has to work correctly in order for those zealous cops to show up at your door, AT ALL, nevermind in enough time to "protect" you.
10. If the cops in your town AREN'T zealous...
If none of this were true, nobody would have bothered to write a book called "Dial 911 and Die".
When you're in danger of deadly force RIGHT NOW, be prepared to defend YOURSELF, or be prepared not to get "protected" AT ALL.