From The Detroit News:
The average response time for dangerous runs in Detroit is 24 minutes from the time a 911 call is received, according to statistics released in April.
Nationwide statistics are not available, but Atlanta, Ga., police have an 11-minute average response time and in Washington, D.C., police respond in an average of eight minutes.
These numbers might show that 911 is working well. In my experience, these averages comprise of two minute reponses to shots fired jobs, and forty-minute responses to burglar alarms going off when stores are opening or closing for the day, or a person is complaining that his tenant won't pay the rent.
But the numbers are for emergency runs only. I think the non-time sensitive calls are not included.