More like 90 minutes. The cops generally show up, set up a perimeter, and wait until the gunfire stops and things have been quiet for a good long time before they start to think about possibly entering the scene.
In quite a few of these recent mass-shootings, people who perhaps could have been saved instead bled to death while waiting for help from the authorities already on the scene, after the shooter was already down and dead.
That’s absolutely sickening. Criminal.