I think it all depends on your body. And how much you smoke. I know people who I didn't think had a snowballs chance in hell of passing a DT. And they passed with flying colors. I think the 30 day rule is just for good measure. But no one really knows how long it takes. I would just drink a lot of water. Make sure my urine was clear. Dilute it as much as possible. That's pretty much all you can do. I would say 13 days is sufficient enough though.