When seconds count, the cops are just minutes away.
I have personal experience with this one. I needed a cop in 1991, when someone was actively trying to break into my home at midnight. I was told they were at least 90 minutes away because of "a big drug bust."
911 operator asked if I had a gun. We were in CA at the time, and I had an unregistered one, so I just told them I could handle it and would call back if it got worse.
"We don't dial 911, and due to the high cost of ammunition, don't expect a warning shot."