Imagine this scenario: You are lying in bed when you are suddenly woken by someone shouting at the top of their voice outside your front door. You look at the clock and it's 4am. A few seconds later you hear the guy screaming (aaaaaarrrrrggggghhhhhhhh!) again and then you hear him try to get in your door.
You decide to call 999, not knowing who it is or what they want.
How quickly would you expect the police to arrive?