My 15 minute ride on my old BMX cruiser, to and from school, or to the store, doesn't get a helmet.
Riding my mt. bike, where ever I ride it, I wear a helmet.
When my fixie is done, I'm not sure if I'll wear a helmet or not. I would imagine sometimes I will, and other times I won't.
I want to buy a lighter helmet though. The one I have is too heavy. Maybe if I get a new one, I'll wear it all the time.