I lock it when I go out (as in out of the accommodation block, I don't lock it when I'm in the house), when I go to bed at night and any time in between when I'm urm... busy. Sometimes it's locked purely because I haven't been out of my room all day if I'm having a lazy day (it's en-suite).
I don't lock it to go to the kitchen etc.
I had a housemate once who NEVER locked her door - not at night, not when she went out... never; Best part? She was paying £15 a month for contents insurance, that, obviously, wouldn't pay out if the room was left unlocked.
Had a giggle at that.