From MikeF:
In LE, I'd suggest the same goes for gang areas or bad neighborhoods. If the populace calls 9-11 and nobody shows up for several hours, then they probably won't feel secure.
Major Marginal can best comment on response times for LE in Chicago; which IIRC can be up to three hours when CPD is busy. Here in the UK there are mandated targets for emergency calls, for LE, Fire and Ambulance - which can exert a profound impact.

In my experience a 911 call is not always an emergency, but as long as the caller gets feedback on when or whether LE / police will attend satisfaction is high. Those who get neither can be a "lost cause".

Communication is the important point and of couse what LE do when they arrive.