i'm a pgy-2 surgery resident....
i was tested during at the start of residency during orientation. i think, unless there are special circumstances, this will be the only time the residency ever tests you. these tests aren't cheap to run and administer, so unless you show up with bong resin on your white coat, or talk incessantly about how high you got off your special stash of mexican cheeba, then i think urine drug screening is something you will only have to deal with once, (if at all).
as a surgery resident though i have actually been tested multiple times... i was stuck four times by needles during my intern year and every time I had to go to the health/safety office and have blood drawn and give urine for a drug/etoh screen. i think this is generally standard operating procedure.
also, if you change insurance companies, you might have to get a piss test as well... though i am less sure about this.
free advice: now that you are about to become a grown-up, and have a grown-up job, you might consider that now is a great time to give up certain... recreational habits.