Lock the door when going out or going to bed. If I'm in or in the garden(out of sight) I'll leave it unlocked.
Ok, I don't lock the back door when I am in my back yard, but if I don't immediately lock the doors after entering or leaving the house then I would probably forget and they could be left unlocked for days.
My front door opens straight from the pavement, so someone could be in and out in seconds if they were cheeky enough to risk it!
When I lived in Hebden Bridge I got chatting to an elderly woman from 3 houses away in our little cobbled backstreet. She was bemoaning the fact that she hardly knew anyone there then. She told me that 70 years earlier she had been great friends with the family that used to live where I was living. They routinely walked in and out of each other's houses without knocking, to borrow sugar, milk etc.!