It will be random. Sometimes they will do it upon you immediately returning from leave, sometimes they won't. I've been tested by myself, I've had instances where my entire shop was tested. (they said it was coincidence, we called BS). Then I've been tested twice in the same week!
We had a guy get busted for drugs, and they sent him home packing as fast as they could. He was booted out of the shop, sent to work scrubbing decks, then at the earliest they could, kicked him out.
Fact of the matter is, if you even experiment with drugs, you're playing Russian roulette with your career. Tell me, how you going to explain to your next employer that you were kicked out of the Marines, for something as stupid as drugs? Think they're going to hire you? Hopefully they will be sympathetic to drug users, even if it was just experimentation.