*
Never. The County with which I have been a reserve deputy and from which I am about to retire is pretty big, and the less populated patrol districts are big. It is not uncommon to be a HARD 30 minute drive from the nearest backup, and not unknown to take an hour to get to some calls. If you don't have the stuff in your car and/or on your person, you may never get it. I also carried 3 spare mags of AR ammo even when I did not have my own carbine. Where I worked 2000-2007 as a prosecutor, and where I am now, it would not be a shock to have no cops within an hour. Large counties, low density tax base. As Pat Rogers says, policing is very regional. Visitors from Chicago/LA/and especially NYPD are aghast at the territory covered. A serious call in NY is likely to have 20 or more cops in 60 seconds.
We did have a shooting in 2005 or so in which so many rounds were expended due to the odd nature of the circumstances, a resupply had to be brought out from the armory. It was admittedly a really weird situation, but for those there, it was a huge test sample.