In my experience I've never been drug tested. I've worked in a restauarant, a call centre, and currently an office job.
Hair tests are very, very rare and as long as it's limited to something like pot and was obviously a long time ago they probably wouldn't care. They don't even hair test in the military... But then again there isn't much point since forces mostly have buzz cuts during active duty/training.
If I were to ever get a urine test though, I know I would pass, even if I had smoked that morning.